I think there is a fundamental change in how Western Art Music is composed around the start of 20th century; the removal of the tonic-dominant relationship

Yes, this is a proposition I reject. Don't worry, I don't expect my claim to be obvious; explaining it would be a rather involved technical discussion. A necessary first step would be the wholesale rejection of the traditional Rameau-Riemann theory of "chord progressions" in the explanation of earlier music, in favor of the kind of approach taken by Schenker and, later, Westergaard.

What do you mean by "party line"? Which part specifically is the party line? Whose party line is it? The party line of musicologists, or the party line of contemporary composers?

All of the above; particularly those of high status.

I agree there is a certain amount of German-centrism in the term "common-practice tonality", but that itself doesn't mean it doesn't exist.

It definitely exists -- but only as a historical cluster of musical works, and not as a theoretical category. From a theoretical point of view (again, my theoretical point of view, which is separated by considerable inferential distance from the memes of traditional music theory), there is little use for a category which includes Bach and early Schoenberg and excludes middle and late Schoenberg.