I did some research and found a clear answer: Probably not in our lifetimes and it's not in Apple's power to make it happen.
Apple cannot interoperate with Google's Google Message <proprietary> E2EE RCS implementation. Google Messages RCS EE2E works only with Google Messages RCS EE2E using Google's <proprietary> authentication. Google Messages RCS EE2E requires one to trust Google's <proprietary> authentication authority. Ditto for Apple, so Apple cannot interoperate with Google Messages encryption and vice versa. It is a technical impasse, not either refusing to work with the other.
They might both be able to use standard PCKS authentication certificates, each acting a trusted certificate authority. Then Apple Message and Google Messages could then interoperate with each other and other RCS E2EE implementations (maybe?).
P.S. IMHO Apple is a trusted authority, scrupulously protecting data privacy. As such they would make an excellent trusted-certificate authority.
In contrast, Google is one of the least trustworthy companies for data privacy. Billions of dollars of their revenu depend on gleaning Gmails, gleaning their search engine, listening to private and intimate conversations 24/7, and then selling some of what they gathered and heard to third parties. They even collect Chrome usage in "Chrome Incognito Mode"!
It's disingenuous of Google to take the high ground, claiming Apple is to blame for not interfacing Apple's proprietary encryption with Googles "open" (proprietary) encryption.
