I think you're right about that in some respects, but also if you acknowledge that the New Mass is valid, was validly promulgated, etc. -- then doesn't that assume that it is in conformity with the immemorial tradition? And if it is, we're back to externals as you mentioned. So as I said, I struggle with this. Clearly the offering of the Son is acceptable to the Father. Yet I think it is empirically clear, using the standards of success that Christ gave us (by their fruits, etc.), the Old Mass is objectively better than the New. So what do we point to to explain why?
I think that the New Mass is valid because of a few sentences in the canon, as with tradition. That leaves open a lot of text around and outside that which are open. But a collect being reformulated to excise a phrase like "despise the world" would be judged by the traditional formulas teaching on faith and morals, and how the new text reflects that teaching.
Example, 2nd Sun of Advent, the ancient postcommunion text:
Repleti cibo spiritualis alimoniae,
supplices te, Domine, deprecamur:
ut, huius participatione mysterii,
doceas nos
terrena despicere,
et
amare caelestia.
The new text:
Repleti cibo spiritalis alimoniae,
supplices te, Domine, deprecamur,
ut, huius participatione mysterii,
doceas nos
terrena sapienter perpendere,
et
caelestibus inhaerere.
So in the context of a penitential season, right after communion, a prayer is changed from "despise [value very little] earthly things, and to love those of heaven" to "ponder earthly things wisely, and to cleave to heavenly things".
Is despise and wisely ponder the same? Is love and cleave to the same? Do they express the same truths of faith? If they do not, and they detract from the expression of faith, then the traditional is protected. If they are equivalent, or even improved base don true development of doctrine (ex. the clarifications concerning Mary), then they may be altered, it would seem.
But even before this, there is the debate of even why we would need to look at reform.
