the Mass of Pius V is the Tridentine Mass, is the only Mass of the Roman Rite, is obligatory on all Catholics of that rite forever and should never change. That's the Law of Quo Primum in a nutshell.
Catholics do not belong to Rites, they belong to Churches -- 24 Churches in the 1 Catholic Communion to be exact.
Churches have Rites, sometimes a single Rite as the Maronite Church uses solely a West Syriac liturgy (one that has, alas, been woefully Latinised ala the Bugini Novus Ordo reforms) and, therefore, bears the name Maronite Rite. Other Churches possess multiple Rites, such as the Latin Church to which most here at CI belong that uses the Roman Rite (in various local uses), the Ambrosian Rite, the Mozarabic Rite, the Braga Rite, and the defunct Gallican Rite (better called the Gaullish or Frankish Rite to distinguish it from the Gallican uses of the Roman Rite in 18th-century & 19th-century France).
Stubborn, what you do is rip
Quo Primum from its historical context, assign to its text a kind of authority that it does not nor cannot posses, and fail to acknowledge that the legislation of
Quo Primum has been derogated several times by Popes subsequent to St. Pius V as well as by the custom of local Churches, such as Paris and Lyon which had uses alternate to that of Rome develop in the centuries immediately following
Quo Primum.
Amongst those who hold Montini to have been a valid Pope, the best of their liturgical scholars understand the 1964 liturgical instruction
Inter Œcuмenici and the 1969 Apostolic Constitution
Missale Romanum to be derogations to
Quo Primum. The lesser of their scholars together with the average Conciliar seminary graduates hold these docuмents, particularly
Missale Romanum, to be obrogations of
Quo Primum. (Since I do not hold Montini to be a true Pope, this latter case is a moot point to me).
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