"Proprietary extended version" is exactly how every open protocol in the world has ever worked. You implement the base protocol so that you can interoperate with everyone, then add your optional extensions with the hope that the extension will be a part of the protocol in the future. And "proprietary" is doing a LOT of work here given that Google's E2EE has been open from the beginning, and is now well documented and will likely be in a future version of the protocol.
Also, do you have a source for "default to Google/Jibe, even if the carrier has a valid server"? I cannot find anything to support that, and the RCS Wikipedia page says "Google Jibe provides RCS service directly if carrier does not" which I parse as "default to carrier server, fall back to Jibe". Not that Wikipedia is authoritative, but absent any other evidence...
It sounds like your preference is that RCS should not have E2EE, so that Google would also have access to the contents of every message? Or that protocol extensions should always be implemented by slow committee rather than "rough consensus and working code". You seem to really hate privacy, dude.