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.