Perhaps, then, St. Pius X incurred the penalties of Quod a nobis and was therefore a non-pope. 
As often happens, the exaggeration of Quo primum comes from a reaction, and then an overreaction, to Montini's Prot rewrite of the Mass. People are looking for some simplistic, easy, legalistic way to invalidate it ... except that SVs just say it's as simple as the fact that Montini was no pope.
To say as you said previous, what one pope binds, another is free to loosen, is to say that the authority of the pope is limited.
This is to say QP of Pope St. Pius V did not have the authority to bind all his successors, which is to say that apparently, Pope St. Pius V attempted to do something which he had no authority to do.
This is what you are saying.
To paraphrase Fr. Wathen; And we say well then, if he did not have that authority then his authority was limited. We say that if his authority is limited, then all his successors' authority is limited also.
We say yes, the authority of the pope is limited, but it is not limited to establishing the liturgy of the Mass for all time, rather, it is limited to where a successor cannot discard this Mass because of a whimsy or a deviation in Catholic belief, and there has to be a deviation in Catholic belief on the part of pope Paul VI who would introduce such a mass as what we have, the Novus Ordo Missae.
All this is to say it has nothing whatsoever to do with the idea that "it's as simple as the fact that Montini was no pope."