Quo Primum concerned a disciplinary matter in the Church, not an infallible teaching on faith or morals. Evidence of this is that an infallible teaching on faith or morals would not—indeed could not—allow for such exceptions as "unless approval of the practice of saying Mass differently was given" or "unless there has prevailed a custom of a similar kind" In fact, the document states, "We in no wise rescind their above-mentioned prerogative or custom."
Such matters of Church discipline always remain subject to future change by equal or greater authority. In light of this, wording such as "in perpetuity" must be understood as "from now on, until this or another equal or greater authority determines otherwise." Pope Paul VI held equal authority to that of Pope St. Pius V. Therefore, changes to the Mass under his authority were licit and valid.