The thing is that we need a lot of different theories, but what we have, mostly, is a pretty well developed theory that deals with music from about 1600 to 1900. This we call the 'common practice' period and our theory is largely based on the music of Bach, Haydn, Mozart, Beethoven and extends to roughly fit the music of Schubert, Chopin, Schumann, Brahms, Liszt, Wagner and Mahler. Before Bach, we try to look at things a bit historically, taking our cue from theoretical treatises of the time. But the workings of modal counterpoint and harmony are not as clear to us as we would like. Since 1900 what we have are a lot of manifestos and propaganda about various progressive or avant-garde composers' methods. They are biased by the needs of promotion so not terribly useful. In the case of, for example, serialism, the theory is really a workshop in how to construct a piece, but is of little use in trying to decide whether the piece is successful or not. I think that valuations are a significant goal of theory. We can certainly use theory to investigate why, say, a particular piece by Beethoven has the kind of effect it does. We should be able to do so with a piece by Webern, if we had a workable theory. But I don't think we do. Vector analysis is not really that helpful as it seems it is neutral between a very good piece and a very bad piece. Yes, I believe that modernism produced quite a number of very bad pieces and it would be interesting to talk about how and why.
I don't think that any music is beyond being examined critically and theoretically, though not all music is worth the effort. But, given a significant repertoire, whether it be of Indian ragas, Javanese gamelan, mbira from Zimbabwe, motown from Detroit or pop from Liverpool, we ought to be able to look at it and see how it works. Same should be true of Ives, Partch or Cage. Though I can see a lot of headaches in the attempt! Heck, I would be happy if someone would figure out how some of Shostakovich's or Stravinsky's music works. Yes, I know that there are various attempts, but a lot of them seem rather unconvincing. It took a long time to really come to understand the Beethoven late quartets, I'm sure it will take just as long to figure out what is happening in the Shostakovich symphonies.
For example, I don't think you can just explain away this piece, the first movement of Shostakovich's 9th Symphony, by calling it "extended tonality". I think you really need to figure out the structure and talk about things like how and why the little duet between piccolo and trombone is so funny. Shostakovich was a funny guy, not the least because he decided to make his 9th Symphony so comic, going against the long tradition, started by Beethoven, of making one's 9th Symphony particularly portentous.
I guess what I'm trying to say is that good music theory includes aesthetic judgment.