There’s nothing stopping Apple from implementing Universal Profile RCS tomorrow, except for their development time. There’s nothing to lobby the GSMA for, it already works.

Now the E2E encryption piece is another matter. Note that’s not committed to coming before 2024 - just standard RCS is.

There won’t be compatibility issues. Google’s RCS service also uses Universal Profile, as it has to communicate with carrier implementations.

The only issue is you won’t get E2EE, but the RCS message will work.