As I said earlier, the Holy Mass, although it contains some disciplinary elements which are subject to change, is in actuality the Church's liturgical expression of the law of what must be believed. And as such it is not reformable as a whole or in those major areas which are intimately connected to the Faith.

The very fact that all of the popes up to John XXIII acknowledged Quo Primum's validity and authority by decrees which explained any small changes that were made. These decrees were then included in the Missals after Quo Primum explaining why the changes were in keeping with the original decree.

All of those popes both realized and acknowledged the binding authority of Quo Primum by their own actions.

Paul VI and Pius XII's protege Bugnini contravened directly Quo Primum by changing the substance of what must be believed by changing what is to be prayed.

The undeniable and irrefutable proof of this is the rapid loss of belief in the Real Presence of Our Lord, the general acceptance of salvation outside of the Church.

Feed your faith indeed.....

Stubborn,
Practically speaking, Pope Pius X was bound by Quo Primum, up until the point where he made changes to it.  In theory, yes, he was not bound by it.  Just as Paul VI wasn't bound and just as Francis isnt bound either.

Just as in American politics, you could say that, in theory, Congress isn't bound by ANY law, because they could pass new ones.  But until they do so, they are bound.  

I like the way Fr. Wathen explains it in The Great Sacrilege:

By the decree Quo Primum, Pope St. Pius put the Mass in the hands of his successors for its protection, in an effort to do all that a Pope might do to establish its form as inviolable and unalterable. He sought to strengthen his law further by binding all Catholics, under pain of mortal sin and the threat of excommunication, to refuse obedience to anyone, even to a Pope, who should attempt to alter the Missal substantially. Though no Pope may bind by law his successors, because of the moral principles involved, all Pope are bound to obey the prescriptions of Pope St. Pius' decree as anyone else, only more so. The one exception to this truth is that, whereas no one else may make the slightest change in the rite of the Mass, a Pope may licitly make incidental and minor ones which may improve it and be of benefit to the worship of the faithful.

    Recall that, as we have seen, Popes are not infallible in the exercise of their legislative power; they are capable of enacting both foolish and bad laws, of commanding that which is foolish and that which is sinful. Quo Primum cannot remove the possibility that, at some future time, a Pope may attempt legally to alter the Mass radically, or even to abolish it. Its main concern is to repose the Mass in the hands of him who is least likely to because it or to allow others to do so.

    Neither when the great Pope issued this decree nor ever since, until very recently, has anyone questioned its validity. Many today, not understanding it clearly, have presumed to disregard its uncompromising language and claim that, as a "merely ecclesiastical law," it could be abrogated by any of the successors of St. Pius. Perhaps they will be checked somewhat in their offhandedness by being challenged to find an explicit admission from Pope Paul VI that he considers this law either revocable or to have been revoked by himself.

    It can also be said that no one of any stature has ever suggested that the Saint was over-reaching his Papal authority by codifying the ritual of the Roman Mass, or by doing so in such apodictic terms. No one was startled or surprised when he issued Quo Primum, and the Church in his day accepted the Missale Romanum without quarrel or difficulty. It is only since the issuance of the "New Mass" of Pope Paul VI that many Catholics, particularly priests, have begun to question its irrevocability. They have done so out of their need to reconcile Quo Primum with the "New Mass" and with the murmurings of their own consciences. Not being able to do so honestly, they argue that, indeed, Pope St. Pius spoke extremely; his words should not be taken literally. In other words, he made a mistake.
 
    If these people would only study the decree, they would realize that there is no reason for them to proceed in such a fashion, nor will it do them any good, for the simple reason that its irrevocability is intrinsic to the nature of its object and purpose. Its object is the Holy Mass, which it seeks to give a final and definitive form, and its purpose is to provide maximum protection for the Mass in order to keep it doctrinally pure and liturgically inviolable. Its method is to consign the Mass and the Missal to the hands of the Supreme Pontiff only. He alone may make whatever accidental changes and adjustments which future circuмstances and the wisdom of experience dictate; he alone and no one else.
   
 Quo Primum takes for granted that neither the Pope nor anyone else may alter the "Missale" radically or replace it completely, for to do such a thing would necessarily violate the Mass itself and contradict all the traditions which gave it its form. There has never been a time when a Pope or anyone else had the right to design or create a Mass, since the formation of the Mass was the work of the Church over the centuries. A Mass must have evolved from the traditions of the Apostles themselves.