@stray_dog_—
Constructive geometry (the stuff with compass, straight-edge, and stylus) doesn't use trigonometry at all, let alone mess with trigonometric identities. Trigonometry was introduced in the context of Cartesian coördinates. And, while trigonometry is normally taught as a part of geometry as such, extensive drilling on trigonometric
identities is associate with the calculus, the underlying geometry of which does not generally arise to the surface in the use of these identities.
That said, I would at least agree that a cultural inertia plays a rôle in the continuation of that drilling.
I doubt that there is a good case for replacing the teaching of these identities with that of linear algebra. I did quite well in the linear algebra courses that I took, but the only time that I've used linear algebra outside of school is in critiquing a book and a paper that used it, and each of those works was pretty worthless. (The paper was a bit more worthy than the book, but was itself a critique of the book, and missed the important issues as the author of the paper flounced-around with linear algebra.)
That's not to say that linear algebra itself is worthless. For example, as you perhaps know, it is used to design airplanes that don't just shake apart in flight. But it is another field such that a society profits mightily from a few people knowing it, but such that the marginal value of teaching it to as many people as it has been taught is negative.