RCS is a modern replacement for SMS/MMS with end-to-end encryption (E2EE) support via MLS. We have SMS/MMS so we need RCS to replace those. Similarly, carrier-based calls are there and should get replaced with a newer standard with E2EE.
RCS is what GMS Android and iOS provide so that's what people need to talk to non-GrapheneOS users.
Which non-carrier-based messaging app or protocol do you think we should include and why? What happens if we decide that's no longer the best one wand want to get rid of it? Apps included in the OS cannot be easily removed but rather only disabled by default for new installs and phased out for new devices. Otherwise, users would have their working setup break.
Yes I understand your point re: interoperability, and I think you've made a convincing case for RCS.
I think in the short term, you should take a look at e.g. Cheogram, Conversations, and Element and consider ways to enhance integration. Address book integration, automatically extending contact do not disturb exceptions, and maybe even dialer integration.
In the long term, it would be best to try to guage antitrust regulator attitudes wrt to protocols like XMPP and Matrix. In the end, they are the only ones who can really force mass adoption of an open protocol. But alternative OSs having native support and integration for such messaging protocols can tip the scales and be used as evidence of the protocol's viability and legitimacy.