Given that there's a 'Disable RCS' toggle (and a 'Resend as SMS' toggle for that matter) that seems to re-enable SMS and eliminate the RCS weirdness, this doesn't really seem to be true. I guess it could be in the future when carriers disable whatever path SMSes are currently going through, leaving you only with RCS that might still be borked.

I'm not entirely sure what you're claiming here, but broadly speaking no, this is not correct.

RCS is, by spec and in practice, intended to fall back to sms/mms if it doesn't work for some reason (e.g. you're roaming and not connected to your carrier. or have opted out. or they're having an outage. or...).

And there's an opt-out (partly because it kinda requires syncing your contacts to the RCS servers... technically only for "online presence" and for any individual you contact to check their RCS status (which is completely reasonable) but do you know where that presence toggle is in Google Messages? I don't).

The fallback is not really automatic or anything, RCS's feature-set is gigantic and allows senders to have far more control over the message's presentation (https://developers.google.com/business-communications/rcs-bu... currently has visual examples of this). It's rather clearly a "built for businesses" system, at least in part. But "RCS might not be available" is very much a core expectation for the stacks as a whole - the world is a big place, and there are many old phones and out of date apps, even if every carrier gets on board. (this is very likely one of the reasons why everyone's just piling into Google's stuff)

If they ever get things working, they might try to force it everywhere, but that's probably like a decade or three away at a minimum. Accurately predicting industry and legal trends on that kind of horizon is basically impossible. They might be planning on it (I have no evidence either way), but achieving is an entirely different matter.