John R and the Maestro have offered two articles on the bull Quo primum tempore well worth reading here and here. What they write is a liturgically driven objection to the reason most traditionalists given for opposing the Pauline liturgy, that it violates the Pian bull, and instead they make the case that the older liturgy more perfectly reflects the Catholic tradition and the Catholic concept of tradition.
Like John R and the Maestro, I rather doubt anyone really thinks Quo primum tempore is an inviolable dogma or irreformable act of legislation that no future pontiff can change. The liturgy has always and everywhere been subject to variation and the pope, as the bishop of Rome, can alter the Roman liturgy. Some other influence must dissuade a pope from fiddling with the Church's worship than a pseudo-dogmatic bull. Does anyone doubt Guiseppe Sarto was pope? He violated the bull Quod a nobis, issued by the same St Pius V two years before Quo primum tempore and which uses the exact same language in restricting alteration and threatening punishment upon agents of change. Every kind of traditionalist from the "continuity" crowd to the sedevacantists accept the 1911 reform and the papacy of Papa Sarto. It would be intellectually inconsistent to reject the Pauline liturgy while accepting the Pian reforms.
Another issue is that of language. Phrases in Quo primum tempore like "we declare and ordain" are often associated with the language of papal definitions and have been since Vatican I. This is understandable now that such language is restricted to binding pronouncements, but it was not always such. This kind of language can be found in any number of papal and conciliar documents from past times whenever a pope, bishop, or council wanted people to follow a given idea or program. It was not necessarily dogmatic teaching. How can a document publishing a book be dogmatic? Also, phrases such as perpetuis futuris and ad perpetuam are often translated as "forever" and "in perpetuity," which carry a strong weight in English. Could the actual use of these phrases not mean something more like "indefinitely" or "on-going"? Urban VIII, S Pius X, and Pius XII seem to have felt no scruple about making changes to the existing liturgy and often supplemented their changes with additional documents (OK, Pius X tossed Quo primum tempore equivalent Quod a nobis and outright replaced it). The behavior of the popes seems to confirm a weaker understanding of the original text.
Quo primum tempore and Quod a nobis reflect a larger pattern of centralization that I have discussed in my Reform of the Roman Rite series here and in a post reacting to some of Fr Cekada's ideas here. The real reason to support the old rite, of whatever vintage, is that it is tradition. It is a traditio, a passed-on thing from our ancestors in faith. Local rites arose because our fathers in faith received what the Church had given them and they allowed it to flourish within their own culture and with their own enthusiasm for the faith. They took the Roman seed, planted it everywhere, and watered it until a great tree of tradition grew, with the Roman trunk and many unique branches. The horticulturist managing the greenhouse of the Church can erect a sign reading "No one may ever touch this tree ever", but he has no way of ensuring the next person with his job follows suit.