Okay, since you asked. I believe that music theory is much bigger than formal articulations of music theory. Anytime any musician or listener has any expectations of music at all, that is music theory. https://twitter.com/Top40Theory/status/1300472733957066752

In other words, unless you are generating music by random chance or listening with complete passivity, music theory infuses every aspect of the experience.

The purpose of music theory is to help musicians make choices, and for listeners to make sense of those choices.

The thing that is customarily taught as "music theory" in American schools is really "one set of approaches to the harmonic and formal conventions of the Western European aristocracy in the eighteenth and nineteenth centuries." This is a tiny subspecialty of actual music theory.

I guess you could draw a distinction between the implicit music theory of, say, your typical self-taught rock guitarist or beatmaker and the explicated music theory of, say, me explaining how their music works. I don't see that distinction as helpful.

That said, there is value to articulating various music theories, because it can save other musicians and listeners a lot of tedious trial and error. "Oh, there's a name for that thing I like, and I can do it systematically! Nice."

Any attempt to come up with a final, definitive or universally valid music theory is doomed to failure and should be viewed with suspicion. Here in the US such attempts are just about always thinly veiled white supremacy (hello Schenkerians!)

Probably it would be more accurate to rename the field "music theories."

As to how we evaluate the validity of a particular music theory: does it help anyone in their creating, understanding, or enjoyment? Then it's valid. To an extent.

Let me be more specific. I just finished making a track for my dissertation mixtape. Here's a partial list of music theories I used:

Things sound good when you repeat them. Things sound especially good if you repeat them in groups of four. It sounds good to have those groups themselves repeat in groups of four, but only to a point, after a while you have to have some large-scale asymmetry or it gets boring.

You can create pleasing rhythmic dissonance by tempo-aligning rhythms in the same meter with different amounts of swing. You can have gradual tempo changes or abrupt tempo changes as long as everything is steady on either side of the change.

If you so much as vaguely align speech with a beat, listeners will assume that all of it is aligned with the beat intentionally.