1.  I do not intend to start an anti-sede feud.  Most people on this thread (at least 2Vermont should know because we've debated many times) that I'm partial to many sede arguments.  So i'm making an honest attempt at civil debate.
.
2.  "Massively altered" is not the same thing as "essentially" altered.  Pope St Pius X massively altered the breviary but the essence of it was not changed.  The new Holy Week, in the opinion of many Trad clerics, is not perfect but it's also not substantially different.
.
3.  An argument can be made that the Holy Week rites are more human/church created than divinely inspired (i.e. they aren't the mass).  The liturgy (outside of the essential, Apostolic parts of the Mass) organically developed over the centuries.  It can be changed quite a bit, in theory.

Pax, I mean no disrespect, but do you have any idea of what was changed? It is was profoundly altered! I started a thread recommending a book I just finished, Born of Revolution by Dr. Carol Byrne. I have read many books on the changes of the common Mass, but this book goes through minute detail of the changes in Holy Week, and it is astonishing. Comparing Pope St Pius X's changes to this is dismissive without knowing the real differences of the two examples. It was without a doubt substantial in difference. 
There has been a significant movement from quite a few camps to the pre 1955. The only sedes that accept the 1955 are the CMRI and very few independent priests. All of the SSPV, all the SGG, all of the ICKSP, some of the FSSP, and quite a few independent priests, 2 of which I receive the sacraments from from time to time (non sede), say the pre 1955. 
The acceptance of the 1962 missal is an accident of history. It was decided by +Lefebrve as a concession to the conciliar hierarchy. The 1962 missal was only in official use for about 2 years. Most of the SSPX outside of France before the 1980's said the pre 1955. 

but this book goes through minute detail of the changes in Holy Week, and it is astonishing.

Ok, but it's not a change to the missal nor a sacrament.  So, it's not a change to anything of Divine origin.  So, it's a change to a human part of the liturgy, which the Church is allowed to do (for better or worse).  Christ gave St Peter the power to "bind and loose".  In all things non-Divine, the pope can change things.  Just like Pope St Pius X massively overhauled the breviary (in all non-essential, non-divine areas).

I thought that Quo Primum forbid the introduction of New Rites into the existing Missal. The new Holy Week is exactly that.

We have a winner!

I thought that Quo Primum forbid the introduction of New Rites into the existing Missal. The new Holy Week is exactly that. It is as much against the law as what you state about the 1969.
Pius XII did not "slightly change" Holy Week. From Palm Sunday to Easter Vigil, it was massively altered.

The Holy Week changes were not a "New Rite".  It was a change to an existing rite.

Interesting excerpt from a longer article of Michael Davies rebutting a priest, who claimed a pope can altar the 1570 missal, just as several other popes had:

"I have already accepted that Quo Primum did not preclude subsequent popes from revising the Missal, but not one of the popes cited by Father Hinwood introduced a single change which affected the ethos of the traditional Missal in any way at all. On the eve of Vatican II the Missal used within the Roman Rite was clearly and unmistakably the Missal of St. Pius V [even Holy Week?], the Ordinary of the Mass was identical, final blessing included! I own a Missal printed in 1577 and have verified this. I have already analyzed the alleged revisions mentioned by Father Hinwood; they are cited frequently by apologists for the liturgical ʀɛʋօʟutιօn (PPNM, pp. 10-13). I have also included the full texts of the more important docuмents in Appendix II to Pope Paul's New Mass.

Father Hinwood cites the "reform" of Pope Clement VIII as a precedent for changing the missal. Read the Brief cuм Sanctissimum and you will find that this Pope's principal concern was that changes made in the Missal in defiance of Quo Primum should be corrected, and that, apart from some minor rubrical improvements, the Missal should be restored in every respect to the text authorized by St. Pius V. He commanded that Missals which deviated from the text published by St. Pius V "be banned and declared null and void and that their use be disallowed in the celebration of the Mass, unless they be entirely and in everything emended according to the original text published under Pius V." How can Father Hinwood possibly expect to be taken seriously after citing Pope Clement VIII's reform of the Missal as a precedent for the ʀɛʋօʟutιօn of Pope Paul VI? He presumes, no doubt, that few if any of his readers will so much as heard of, let alone read, cuм Sanctissimum, or any of the docuмents relating to his list of "revisions." I suspect very strongly that he has not read them himself. "Pope Pius X brought in revisions," he claims. Quite true—he changed the musical notation! "Pope Leo XIII added the prayers to be recited at the end of a said Mass." He did, indeed, but this did not affect the text of the Mass in any way at all. Does Father Hinwood seriously expect us to accept that the recitation of the Leonine Prayers after a Low Mass provides a legitimate precedent for the ʀɛʋօʟutιօnary onslaught upon the Roman Missal which gave Father Gelineau ample justification for claiming that the Roman Rite had been destroyed?" http://www.angelusonline.org/index.php?section=articles&subsection=show_article&article_id=741 

Question regarding the bolded font above:

Did Pius XII/Bugnini "introduce a single change which affected the ethos of the traditional Missal in any way at all."

Many (such as Fr. Stefano Carusi) would affirm the drastic and ʀɛʋօʟutιօnary nature of the changes, such as: Introducing the versus populum posture; introducing the vernacular; introducing carrying the sacred action into the body of the church; washing the feet of laymen; moving the vigil from daytime to evening; introducing congregational prayer; Communion on Good Friday; and doing all this on the basis of an archaeologism which Pius XII had just condemned a few years earlier.

 

Question regarding the bolded font above:

Did Pius XII/Bugnini "introduce a single change which affected the ethos of the traditional Missal in any way at all."

Many (such as Fr. Stefano Carusi) would affirm the drastic and ʀɛʋօʟutιօnary nature of the changes, such as: Introducing the versus populum posture; introducing the vernacular; introducing carrying the sacred action into the body of the church; washing the feet of laymen; moving the vigil from daytime to evening; introducing congregational prayer; Communion on Good Friday; and doing all this on the basis of an archaeologism which Pius XII had just condemned a few years earlier.

Bravo.  

Nope.

It was a Novus Ordo.  

Anyone attending Palm Sunday, Holy Thursday, Good Friday, or the Easter Vigil until 1951 would have been very confused in 1956 (and would have missed the Vigil altogether).

I agree that there were changes, but it's not like the whole missal was revamped into a New Rite ...like the 1969 Rite.  

When I say New Rite, I'm talking about a New Rite like the New Rite of Episcopal Consecration (compared to the Old Rite).