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Oh boy. Music theory, as such, has no "rules," and so of course Bach did not invent them. Music theory is a descriptive enterprise, which aims to make sense of music as composed/performed/enacted by humans. (I have a PhD in music theory.) Bach's chorales were functional music for the Lutheran church, and to the extent that they form any sort of "rules" in music theory, it comes from the fact that they have been used to teach harmony for a long time (since at least the 1940s, as evidenced by this article). The reason for that isn't so much that they're prime examples of Western common-practice harmony, but rather that they have a homogeneous texture that's easy to use in classrooms, because they're easy for one person to play at the piano or for students to sing. Recent music theory pedagogy has largely been moving away from the reliance on Bach chorales to teach harmony, especially as music theory has taken a broader perspective on what music we should be studying anyway. Studying the Bach chorales is just fine if you want to know about how Bach used harmony, but there's a whole lot of music in the world, and there's no meaningful sense in which Bach's music intrinsically defines a set of rules any more than Mozart's or Clara Schumann's or AC/DC's or Meredith Monk's defines a set of rules. |
Although as a PhD you obviously know the subject much better than I do, I'll venture a tentative dissent, mostly because I'm curious what your rebuttal will be.
As an undergrad I took a two-course sequence in music theory, I loved most of it. I still remember nearly everything I learned, and twenty years later I was able to more or less reproduce one of my compositions from memory.
Our professor promised us that at the end of the semester we would compose four-part chorales and sound like Bach. I flat-out didn't believe him, but indeed I was able to compose something I was happy with. Overall, at the end I felt like I to a large extent I understood music -- much more so than I initially believed to be even theoretically possible.
By the end of the second semester, as we got into the twentieth century, the "rules" got broader and broader, and the course seemed to get vaguer and vaguer. Although I love twentieth century music, I stopped enjoying the class: different compositions had less and less in common, and there didn't seem to be any large-scale "theory" to be explained. Every piece had its own theory, and I didn't feel like I "understood" anything at all. Rather than attend class, I'd rather just go to a concert hall.
I certainly agree that there are a tremendous variety of musical traditions, many of which arose in places other than Western Europe. Calling it "music theory" is a disservice, when what's being explained is the theory of a single one of these traditions. Nevertheless, I'd rather study one of them in depth than take a broad survey.