And what, then, of Quo primum?

Quo Primum: Could a Pope Change It?
QUESTION: During a recent argument with a Novus Ordo friend, she told me that (according to her priest) popes can change whatever they want, as long as it is not dogmatic. We were discussing Quo Primum. I told her that it was forever, but she said that even if the pope said forever another pope can change it. What would you say to that?
REPLY: On this point, shes right.
A (true) pope is the supreme legislator for ecclesiastical law and has the power to change ecclesiastical laws enacted by his predecessors. Quo Primum was an ecclesiastical law, and a true pope did indeed have the power to abrogate it or modify any of its provisions.
The forever clause was merely a type of legal boilerplate common in all sorts of papal legislation.
In the 1960s faithful Catholics seized upon this language as a justification for disobeying the new liturgical legislation while simultaneously recognizing Paul VI as a true pope. This was unfortunate, because anyone who knows a bit about canon law can refute the argument very easily.
The argument also obscures the real reason for adhering to the traditional Mass and rejecting the New Mass: The old rite is Catholic. The new rite is evil, inimical to Catholic doctrine (on the Real Presence, the priesthood, the nature of the Mass, etc.) and a sacrilege.
If you send me your postal address, though, Ill send you a consolation prize: some copies of a booklet I wrote, Welcome to the Traditional Latin Mass, that compares the old Mass and the New Mass.
Give a copy to your friend and tell her to give it to her priest. That should keep him busy for quite awhile!
QUESTION: So you are saying that a real pope can change a Papal Bull decree that another pope has made in perpetuity? Why would a pope decree something for all time, if another pope could change it?
REPLY: If it was a disciplinary Bull (establishing a church law), yes, another pope could change it.
The language was simply a standard formula in church legislation that referred to one of the qualities a law is supposed to have: stability.
Frequent changes in laws harm the common good because people do not know how to act — hence, laws are supposed to be relatively stable. But a human legislator (unlike God) cannot foresee all future circuмstances, so his successor has the power to change existing laws if he decides the circuмstances warrant it.
This reflects a general principle in law: An equal does not have power over another equal. No pope who used perpetuity in his disciplinary decrees understood the term to mean that no future pope could ever amend or replace his legislation.
And popes did in fact change some of the provisions of Quo Primum, even before Vatican II. In 1604, for instance, Pope Clement VIII issued new regulations for the Blessing at Mass, and in 1634 Pope Urban VIII changed the wording of the Missals rubrics and hymn texts.
Traditionalists should stop using the Quo Primum argument. It’s a canon law urban legend — as in “alligators in the sewers,” rather than Urban VIII!

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The idea that it binds future popes is basically a myth. Here's what Fr. Cekada wrote about it:

Key part:

50. The sacred liturgy does, in fact, include divine as well as human elements. The former, instituted as they have been by God, cannot be changed in any way by men. But the human components admit of various modifications, as the needs of the age, circuмstance and the good of souls may require, and as the ecclesiastical hierarchy, under guidance of the Holy Spirit, may have authorized.