All Apple has to do is what they already do with "Sign in with Apple". Just require everyone who uses third-party payment to also offer Apple's services.
That one little step would avoid the regulation they're going to be hit with.
The way I see it, unless you're developing exclusively for Apple platforms, it's not worth the effort to build something with Metal. There are translation layers like MoltenVK that can help, but that still requires a lot of effort for what is likely a very small market segment.
That one little step would avoid the regulation they're going to be hit with.