well (if this is not what you mean) music theory should be called "western music philosophy (or language)". "Theory" implies it is scientifically proven as in a scientific theory. While there is a lot of science in music theory, almost every philosophy and even fiction has scientific/historical facts in it. But often what people argue about is the philosophy part, and not the science part. And within western theory, there are endless sub-philosophies as well, which spawn controversial topics.
For instance IMO functional harmony is overrated and overused (especially complex harmony), even though it is a basic tenet of western theory. IMO the best music is written with melody and rhythm as the base, and harmony should be an afterthought if used at all. But people strongly disagree and so much music is written around chord changes. yes, all completely subjective, but it is surprising how emotional people get with such topics. And then the "you are presenting your opinion as fact" and other strawman and empty argument nonsense comes, when people don't like your subjective opinions but have no other way of attacking you.
