Fallbacks can be layered. For a company that claims go all-out for its consumers privacy, using SMS as the default fallback in the 21st century is pretty sus.
There’s over a dozen different versions of RCS, hand who knows which carrier supports which parts. It’d be more fall-through (the cracks) than fall back.
I personally think they should adopt something brand new and not evil. I'm not saying that RCS is evil, but from what I can tell, while it has federation, Google seems to be gaining a monopoly on RCS (sorry, I don't remember what article I read that in). My pick would be something like Matrix or even XMPP to make sure that there's a definite standard that is federated by design; however, you'd have to trust your carrier/software to not dump your messages onto Google's server instead of using a different server.
All that Google is asking is for the fallback to be at least in the current century, is that too much to ask.