Now certainly I agree that the fathers at Trent *would have* defended the validity of non Tridentine Rites (such as the Eastern Rite of Mass) but I doubt they were even considering the idea of someone saying *only some* rites are valid. Rather they were condemning protestants who were invalidating all rites of mass.
Correct. Trent was, as were all councils, concerned with doctrine. Trent was concerned with the Protestant heresies and protecting the Faith.
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Quo Primum was concerned with the liturgy; it is an "internal memo" of the Church, to deal only with its liturgy. It had nothing to do with Protestantism, except for the fact that the protestant heresy highlighted the need for uniformity. While the liturgy is an expression of doctrine, the liturgy is a separate entity.
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Quo Primum specifically allows Eastern rites that were over 200+ years old at the time of 1571 to continue, since these rites are part of the Traditions of the Church. Any rites that were less than 200 yrs old at the time of 1571 were deemed "new" and had to conform to the Tridentine norms. The whole purpose of Quo Primum was to streamline the Latin Rite so that the Church could pray "as one", using one rite. The old, ancient Eastern rites were not affected by Quo Primum, because they are Apostolic in nature. They are also substantially and essentially the same as the Latin rite, and their use is much, much smaller than the latin rite, so there was no need for correction, since the Eastern rites did not experience the same degree of "creativity" as did the Latin rite.