You’re not getting RCS into any proper messenger.

RCS is garbage that’s wholly hardware-bound (your phone/IMEI/Sim) which is pretty much antithetical to any modern (i.e. Post 1994) design.

Why would I want to use a “protocol” that’s not network-agnostic (just like SMS)?

Protocols like XMPP/Jabber predate RCS by close to 20 years, and already did everything RCS will eventually do, but it did it in 2010. Except be bound to your phone number (and encryption didn’t occur until more recently).

SMS at least was an honest attempt to utilize the empty space in cell management frames. RCS is nothing more than an attempt to keep people using their phone numbers/IMEI as an ID when far better systems already exist. What possible rational can there be to develop a protocol that’s bound to hardware identifiers in the 21st century?

A even better question to ask is why these organizations are abandoning Jabber/XMPP when they were using it for their own chat programs since 2000, and it’s had steady development over that time? (Hint: Jabber/XMPP is an open protocol, doesn’t lock you into any system/app/service. Anyone can use it, any client can communicate with any server over any network. RCS can’t. I currently run 4 different clients using the same accounts, on 5 or 6 devices, and they all stay in sync nearly instantly, with encryption).