Yes, there are legal docuмents where certain words and terms can have certain legalistic meanings other than what the word normally means, and then there are official Church docuмents that are meant to mean what they say, to be understood as they are written outside of a court of law or lawyer's office, where 'in perpetuity' and 'forever' are to be understood literally, to mean what they say by design so that there is no mistaking the message.

Personally, I do not believe that Quo Primum could ever be legally abrogated because of what it pertains to, the eternal sacrifice. I believe PPV had that authority to bind even popes, and knew why he bound us forever, and knew well that he had the authority to do it. Many other trad priests believe(d) the same, Fr. Altenbach is on record preaching it....https://tinyurl.com/mrxc4k5n  Heck, even the Dimonds believed it, not sure but they probably don't believe it any more, but that's only because there's a whole lot more to sedeism than just a vacant chair.

10. No one at all, therefore, may infringe this docuмent of our approbation, re-introduction, sanction, statute and derogation of wills and decrees, or by rash presumption contradict it. If anyone, however, should presume to attempt this, let him know that he is destined to incur the wrath of Almighty God and of the blessed Apostles, Peter and Paul.

6. In addition, [by this Our Constitution, which is to remain valid in perpetuity We] enact, determine, decree and define:-] that if ever at any time it shall appear that any Bishop, even if he be acting as an Archbishop, Patriarch or Primate; or any Cardinal of the aforesaid Roman Church, or, as has already been mentioned, any legate, or even the Roman Pontiff, prior to his promotion or his elevation as Cardinal or Roman Pontiff, has deviated from the Catholic Faith or fallen into some heresy:

(i) the promotion or elevation, even if it shall have been uncontested and by the unanimous assent of all the Cardinals, shall be null, void and worthless;

(ii) it shall not be possible for it to acquire validity (nor for it to be said that it has thus acquired validity) through the acceptance of the office, of consecration, of subsequent authority, nor through possession of administration, nor through the putative enthronement of a Roman Pontiff, or Veneration, or obedience accorded to such by all, nor through the lapse of any period of time in the foregoing situation;

(iii) it shall not be held as partially legitimate in any way;

(iv) to any so promoted to be Bishops, or Archbishops, or Patriarchs, or Primates or elevated as Cardinals, or as Roman Pontiff, no authority shall have been granted, nor shall it be considered to have been so granted either in the spiritual or the temporal domain;

(v) each and all of their words, deeds, actions and enactments, howsoever made, and anything whatsoever to which these may give rise, shall be without force and shall grant no stability whatsoever nor any right to anyone;

(vi) those thus promoted or elevated shall be deprived automatically, and without need for any further declaration, of all dignity, position, honour, title, authority, office and power.

https://www.catechism.cc/articles/Pope-Paul-IV-Ex-Apostolatus-Officio.htm

Father Cekada at one point made a solid point about the terms like "in perpetuity", where it simply means that it's a permanent order, would remain in force even after the pope's death (unless abrogated), vs. some temporary or time-bound edict.

"in perpetuum" in Latin just means permanent or lasting, and not the same as the Latin "in saecula saeculorum".

General principle of all law is that an equal cannot bind an equal, so one pope cannot bind future popes, except where it's a matter of Divine or natural law, but then it's not the pope doing the binding, but God Himself.

General principle of all law is that an equal cannot bind an equal, so one pope cannot bind future popes, except where it's a matter of Divine or natural law, but then it's not the pope doing the binding, but God Himself.

As far as Quo Primum goes it's actually moot point, because contrary to the sedes in this thread, we know with certainty that Quo Primum was never abrogated, if for no other reason than because pope Benedict XVI told us that Quo Primum was never abrogated in his Motu.

Yes another dishonest red herring, along with a begging of the question.  This thread has nothing to do with Ratzinger but the hypothetical question of whether a legitimate Pope in principle could make an addition or change to the Canon despite Quo Primum ... and not whether Montini et al. had abrogated it.  You also beg the question that Ratzinger was even the Pope and that he was telling the truth rather than making the statement as a tactic to suck Traditional Catholics back into the Conciliar Church (as Ganswein later admitted he had intended to do).

Benedict XVI did not refer to Quo primum at all in SP.  It is not in the text or even in the footnotes:

https://www.vatican.va/content/benedict-xvi/la/motu_proprio/docuмents/hf_ben-xvi_motu-proprio_20070707_summorum-pontificuм.html