15 years and it's still problematic? Sounds like something that just needs to be let go.
Stop pushing this garbage that's tied to hardware/sim/phone number on us. What value is that for the end user, seriously? Why would an end user today want a messenger that's tied to a phone number?
Fully-functional, cross-platform, network-based, open-source instant messaging has been available (even on mobile) since 2009, maybe earlier.
What I always ask about RCS: who benefits today from a messaging system that's hard-bound to a phone number?
You don't know what you're talking about. Samsung, Verizon, ATT, and who knows who else all had their own proprietary RCS apps that were not compatible to any others.