Specifically Googles implementation is a replacement, but then it’s the exact same situation we’re in now, just with Google instead of Apple.
No, because I can dig up official documentation for googles implementation. Where’s Apples iMessage documentation?
RCS is a replacement for SMS, it was intended for carriers to implement it as is standard in the EU. In the US however, the carriers have infamously resisted calls to get off their ass and implement it. Even Google was calling on carriers to do it for years, they only came out with their Jibe platform because the carriers weren’t doing their jobs