Whoever it is that has told you that the Mass is a mere matter of "discipline" was lying to you. The fact that you have believed it is another matter. It is false, and on that principle alone we should begin, if any sense is to be made of this disagreement.
The Bull Quo Primum was not an off-the-cuff, disposable document that any future pope can choose to ignore. Could the current pope overturn the dogma of the Immaculate Conception? What we have today is a pope who is actively countenancing the public distribution of Holy Communion to public heretics. Such an offense against the Faith of Catholics would have been sufficient for public outcry in ages past. Why not today?
In case you missed it, because of the Phenomenolgy of JPII, objective, moral acts are no longer being used as the criteria upon which moral judgments are made. According to this bad philosophy, the subjective sentiment of the individual is the touchstone of right and wrong, and because of that moral evil -- perhaps what Pope Saint Pius X was referring to when he spoke of the poison of Modernism injected into the root of the tree, killing the trunk and all the branches -- we now have a pope and bishops who decline to stand up for truth in the face of objective moral evil in front of them, because in answer to the question "what is truth" their answer is, "we don't know."
The missal that Pope Saint Pius V came out with in response to Trent is essentially no different than the missals that were in use before Trent. That Mass, the Roman Rite, was in its essential elements the same Mass that St. Peter said in Rome 1500 years before that time. If you disagree with this premise, we can go into our respective sources and compare notes. The Traditional Roman Rite found in the missal of St. Pius V cannot be compared to the Novus Ordo on several grounds, one of which is that the N.O. was entirely drawn up out of whole cloth, a new concoction, not based on tradition, but with the intention of making a "liturgy" tolerable for Protestants. The nature of it is not the nature of the Traditional Roman Rite. You can believe what you like, but that does not change the fact of history. Phenomenologically refusing to acknowlege what has taken place does not change the fact of what has taken place. It is a bit like saying that there was no sound when a tree fell in the forest because there was no one there to hear it.
That, by the way, is another maxim of corrupted modern philosophy, because it relies on the observation of an event to define the existence or quality of the event itself. What if a witness was nearby who did not want to believe that he had heard the sound of the tree falling? If he refuses to believe his own ears, who is there to tell him he has made an error? Are we to judge his intention in refusing to report the sound? What if he is crazy, does that discredit his testimony? He might not have been crazy at the time...