Can you site another pontifical document with similar language over a disciplinary matter? (this is a curiosity question, not a challenge)
I've always thought that Pius V did intend exactly what his words said he did, but since he has no power to bind his successors, it didn't matter.
St. Pius V's bull reforming the breviary used similar language. I could see an argument made for the Mass (if the Pope actually had that power), but the liturgy of the Hours isn't even a sacrament, and Pius X was totally unaware of the idea that Pius V's bull "Quod a nobis" tried to do such a thing as bind Pius V's successors. If Pius V had actually tried to do that, Pius X would have had to explain the problem in Divino Afflatu, just like Pius XII had to deal with the Council of Florence in Sacramentum Ordinis.
Therefore, no one whosoever is permitted to alter this letter or heedlessly to venture to go contrary to this notice of Our permission, statute, ordinance, command, precept, grant, indult declaration, will, decree and prohibition. Should anyone, however, presume to commit such an act, he should know that he will incur the wrath of Almighty God and of the Blessed Apostles Peter and Paul. (St. Pius V, Quod a Nobis)
If you look over "Quo Primum", there is never an explicit statement binding his successors. Interestingly, I did just recently come across a Papal attempt to forbid action by his successors:
Thus, moreover, by the authority of the holy Apostles Peter and Paul we entirely forbid that at any time any of our successors from this our permission take or fix a rule for himself or another, because the authority of the ancient Fathers has not promulgated this by order or grant, but too great a necessity of the time has forced us to permit it... (Nicholas II, Council of Rome in 1060, Dz. 354)