What does Quo Primum really say? Here is a breakdown of this law, point by point, which shows that not only does the law permit the True Mass to be said "in perpetuity" (see point 2f) but it also commands that ONLY the True Mass be said (points 2a and 2e), under pain of sin (point 2e).
Most people know of the permissions granted; few understand that this law also commands certain things. These commands are a key point in condemning the novus ordo and the indult as illicit, and therefore, sinful. Quo Primum does not allow any changes, modifications or additions to the missal (point 2d), and it forbids any cleric from forcing any catholic to say/attend a different missal (point 2f). Therefore the indult is illegal because by attendance/use of the indult missal, you are accepting that the novus ordo is ok and legal, which is contrary to Quo Primum (points 2a-2f, all of them).
If you concentrate on Part 2 below, you will know all you need to know about Quo Primum. Part 1 is an intro and an explanation of why the law is being created and Part 3 is just the details of how it will be put in place. Part 2 is the essential law.
Below is the english Translation of Quo Primum, with my commentary in red.
QUO PRIMUM
Apostolic Constitution and Papal Bull
Pius, Bishop, servant of the servants of God, Ad Perpetuam Rei Memoriam:
---Part I : INTRO and REASON for the Law—
From the very first on Our elevation to the chief Apostleship we willingly set our mind and energies and directed all our thoughts to those matters which concerned the preserving of a pure Church worship and We strove, with God’s help, by every means in our power to accomplish this purpose.
Translation: “From the very first” (this is what "Quo Primum" means) moment I became Pope, I have thought about preserving our Liturgy and I have been working to accomplish this.
Since, besides other decrees of the sacred Council of Trent, we were instructed to revise and re-edit the sacred books: the Catechism, the Missal, and the Breviary. Now that, God willing, the Catechism has been published, for the instruction of the faithful, and the Breviary has been thoroughly revised unto the worthy praise of God, so that Missal and Breviary are in harmony, as is but fitting – (for it is only as it should be that in the Church there be only one proper manner of reciting the Psalms and only one rite for the celebration of Mass). We deemed it necessary to give our immediate attention to what still remained to do, viz., to re-edit the Missal as soon as possible.
Translation: The Council of Trent ordered that I revise the Catechism, Breviary and Missal. I have already published a new Catechism and Breviary. (For it is right that there only be one way to pray the Breviary and one way to say Mass.) So I turned my attention to revise the Missal.
Wherefore, We decided to entrust this work to learned men We selected. These men then very carefully collated all their work with the ancient codices in Our Vatican Library and with reliable, incorrupt or emended codices from elsewhere. Besides this, they consulted the works of ancient and approved authors concerning the same sacred rites and thus have restored the Missal itself to the original form and rite of the Holy Fathers. Now that this work has been gone over repeatedly and emended, We ordered after serious study and reflection the work to be printed and published as soon as possible so that all might derive the benefits of this work that was undertaken, namely, that the priests would understand which prayers to use, which rites and ceremonies they were required to observe in the future celebration of Masses.
Translation: I entrusted this work to educated men, who researched docuмents in the Vatican Library and elsewhere. I reviewed their work and I ordered the Missal to be printed, mainly, so that priests would know the correct prayers to use in saying Mass.
---Part 2: TERMS of the LAW---
--2a) What is the law?--
Let all everywhere adopt and observe what has been handed down by the Holy Roman Church, the mother and mistress of the other Churches, and let not Masses be sung or read according to any rite (formula) other than that of the Missal published by Us.
Translation: Let everyone, everywhere observe “what has been handed down by Holy Roman Church” and do not use any other missal when saying or singing Mass, except this one.
--2b) Who does the law apply to?---
This applies henceforth for all future time throughout all Provinces of the Christian world to: all Patriarchates, Cathedral Churches, collegiate and parish churches, secular or religious, both men and of women – even of military orders – and of churches and chapels without a determinate congregation in which Conventual Masses are sung aloud in choir or read privately in accord with the rites and customs of the Roman Church.
Translation: From now on, my law applies, for all future time to … (all Catholics. See list above).
This same Roman Missal is to be used by all, even if the same aforesaid Churches have been authorized, however exempt they might be, by Apostolic indult, custom or privilege, or even if approved under oath or official confirmation by the Holy See, or their rights and faculties guaranteed in any other manner whatsoever.
Translation: My new missal is to be used by all, even if that Church (i.e. rite) has any previous exemption, even from the Holy See.
---2c) Exceptions to the Law---
Only this new rite is to be used, unless in the same Churches from their first institution more than two hundred years ago and confirmation by the Apostolic See, or unless their custom of more than two hundred years’ duration of celebrating Masses in these Churches has been steadily observed – in which cases We in no wise rescind their above-mentioned institute or custom. Accordingly, if this Missal we have seen fit to publish be more agreeable to these latter, We grant them permission to celebrate Masses according to this Missal, with the consent, however, of the Bishop or Prelate or the whole Chapter, everything to the contrary notwithstanding.
Translation: You must only use this rite, unless…(here’s an exception to the law): A) Your Church (i.e. rite) was instituted over 200 years ago and confirmed by the Holy See, …Or B) your Church (i.e. rite) has a liturgical practice of over 200 years – In these cases, I do not force you to use my missal.
However, if the Churches who have exceptions, like this new missal more than their own, I grant them permission to change if they get consent from their Superiors, despite anything I said above to the contrary.
---2d) How does this law affect previous laws concerning the Missal?---
All the other above-mentioned Churches, however, are hereby denied the use of the same Missals, which are entirely and absolutely to be discontinued,
Translation: All other Churches (without exceptions), cannot use their current missals and they must stop using them absolutely and entirely.
while by this present constitution, henceforth valid in perpetuity, We order and enjoin that this Our recently published Missal must never be added to, that none of it be omitted, and that none of it be changed under penalty of Our indignation.
Translation: For by my new law (i.e. constitution), which is valid forever, I order that my new missal must never be added to, changed or omitted from, under penalty of Papal outrage and wrath.
---2e) By what authority is this law created?---
(Wherefore) We specifically command each and every Patriarch, Administrator, and all other persons of whatever ecclesiastical dignity they may be, be they even Cardinals of the Holy Roman Church or possessed of any other rank or pre-eminence, and We order them in virtue of holy obedience to chant or to read the Mass according to the rite and manner and norm herewith laid down by Us hereafter to discontinue and completely to discard all other rubrics and rites of other Missals however ancient which they have customarily followed; and they must not in celebrating Mass to presume to introduce any ceremonies or recite any prayers other than those contained in this Missal.
Translation: Therefore, I specifically command each and every Bishop (….and Church Superior, etc), and I order them in virtue of holy obedience, to say and sing Mass according to my missal and to stop using and discard all other missals, no matter how long you have been using them. And you must not, when celebrating Mass, introduce any ceremonies or recite any other prayers, except those in my missal.
---2f) How long does the law last? Does the law expire?---
Furthermore, by these presents in virtue of Our Apostolic authority We grant and concede in perpetuity that for the chanting or reading of the Mass in any Church whatsoever, this Missal is to be followed absolutely without any scruple of conscience or fear of incurring any penalty, judgment, or censure and may freely and lawfully be used. Nor are (any) Superiors, Canons, Chaplains and other secular priests or religious of whatever order or by whatever title designated obliged to celebrate the Mass otherwise than as enjoined by Us.
Translation: Further, by my Apostolic and Papal Authority, I give permission, for all eternity, that my missal can be said or sung in any Church, it can be used absolutely, without any doubt of conscience or fear of penalty, and it may freely and lawfully be used. Nor can any cleric be ordered to celebrate Mass in any other way, than as We have commanded.
We likewise declare and ordain, that no one whatever is to be forced or coerced to alter this Missal and that this present docuмent cannot be revoked or modified, but remain always valid and to retain its full force – notwithstanding the previous constitutions and decrees of the Holy See as well as any general or special constitutions or edicts of Provincial or Synodal Councils and (notwithstanding) the practice and custom of the aforesaid Churches (established long, immemorial prescription, not, however, if of more than two hundred years standing).
Translation: I also command that no one can be forced to change this missal and that this law cannot be changed or nullified, but will always remain valid and in force.
My law overrides previous laws of the Holy See, any local councils, and the old liturgies of all Churches, except if (as mentioned above) such Churches have rites over 200 years old.
---Part 3: WHEN does the LAW go into effect---
It is Our will, therefore, and by the same authority We decree that after We publish this Constitution and edition of the Missal, the priests of the Roman Curia are, after thirty days, obliged to chant or read the Mass according to this Missal; that those south of the Alps, in three months; and that those beyond the mountains either within six months or whenever the Missal is available for sale.
Translation: It is my will, and by the same Papal Authority I command, that after I publish this law and missal edition, the Roman Curia are ordered to use it after 30 days. Those south of the Alps, have 3 months; Those beyond the mountains either within 6 months or whenever the missal is available.
In order that (the Missal) be preserved incorrupt in the whole world and kept free of flaws and errors, (the penalty for nonobservance for printers mediately or immediately subject to Our dominion and that of the Holy Roman Church will be the forfeiting of the books and a fine of 100 gold ducats payable ipso facto to the Apostolic Camera.
Translation: To keep this missal free from errors, there is a penalty for those who print changes to it. For those printers who are under my authority in the Roman Church, the fine is 100 gold coins, and forfeiture of the missals.
(Further), as for those located in other parts of the world, the penalty is excommunication ipso facto incurred and such other penalties as may in Our judgment be imposed; and We decree by these presents that they must not dare or presume either to print or publish or sell or in any way to accept such books without Our approval and consent or without the express consent of the Apostolic Commissary (to be appointed by Us) in the same locality and unless the said printer receive a genuine Missal to be the model of subsequent printings which must be collated with the genuine Missal and agree faithfully with it and in no wise vary from the Roman Missal with the large type.
Translation: Further, for those printers located in other parts of the world, the penalty is ‘ipso facto’ excommunication and other penalties that I may judge fit.
I order that (printers) must not print or publish the Missal without my approval or without the direct consent of the Apostolic Commissary, who I will appoint for your region, and unless the printer receives a genuine Missal to copy from. The copies you ‘print’ must be compared with the genuine Missal and agree to it faithfully.
Accordingly, since it would be difficult for this present letter to be sent to all parts of the Christian world and simultaneously come to light everywhere, We direct that it be, as usual, posted and published at the doors of the Basilicas of the prince of the Apostles, also at the Apostolic Chancery, and on the street at Camp Fora; at the same time, We direct that printed copies of this same letter be signed by a notary public and made official by an ecclesiastical dignitary and enjoy the same indubitable validity everywhere in every nation as if Our present writing were there shown.
Translation: Because it would be difficult for this letter to be sent to all parts of the world and read at the same time, I order that it be posted at St Peter’s, at the Apostolic Chancery and at Camp Fora. Also, I direct that copies of this letter be notarized and made official, so that the copies would be trusted everywhere, in every nation, as if my present letter were there itself.
Therefore, no one whatsoever is permitted to alter this letter or heedlessly dare go contrary to this notice of Our permission, statute, ordination, command, precept, grant, indult, declaration, will, decree, and prohibition. Should anyone, however, presume to commit such an act, he should know that he would incur the indignation of Almighty God and of the blessed Apostles Peter and Paul.
Translation: Therefore, no one is permitted to alter this letter (i.e. this constitution/law) or go against its permission, statute, ordination, command, precept, grant, indult, declaration, will, decree and prohibition. Should anyone, however, do so, he will incur the wrath of Almighty God and of the blessed Sts Peter and Paul.
Given at Rome at St. Peter’s in the year of the Lord Incarnation 1570 on the 19th of July in the 5th year of Our Pontificate, H. cuмin Caesar Glorierius