Removing the GIL will also make some currently-correct threaded code into incorrect code, since the GIL gives some convenient default safety that I keep seeing

Removing the GIL will also make some currently-correct threaded code into incorrect code, since the GIL gives some convenient default safety that I keep seeing people accidentally rely on without knowing it exists. Unavoidable performance costs and reduced safety equals massive friction no matter how it's approached. It's not a question of "are we smart enough to make this better in every way", since the answer is "no, and nobody can be, because it can't be done". It's only "will we make this divisive change".