Because SMS is horribly limited. 140 chars per message* (less if chars are not plain vanilla ASCII), no support for attachments, group messages, reliable delivery receipts, emoji reactions, etc etc.
* There's a terrible hack called concatenated SMS that strings together multiple messages to build one longer message under the hood, but if any of those parts go missing along the way, the whole thing gets dropped on the floor.
For the proposed use case, you don't need those things, except maybe the 140 character thing, but I've never found that limiting, since phones stitch them together nowadays (and have for the past 15 years?).
Sure, RCS has those functions, but half of them are broken 60% of the time, and you don't need those anyway for bootstraping into wherever you actually want to talk, and for short messaging.
RCS brings nothing to the table if all you need is to tell mum she needs to come pick you up. On the contrary, it might fail you because it tried and failed to send that message over a 4G connection you barely have, rather than sending it as an SMS and then actually arriving. And you're never going to use it for group messages, attachments or with emojis unless its an actual service you intend to use for serious purposes, which is exactly what the comment I was responding to said you weren't going to use RCS for anyway.
I disabled RCS (and iMessage back when I had an iPhone) for exactly these reasons, but still use SMS as a fallback with people I don't actually know and never intend to talk to again, and see no reason to upgrade to RCS even if it wasn't broken, since for my use cases, the extra feature set isn't needed. If I need more fancy features, its for use with people I actually know, and thus people I can get in touch with on not-SMS.
Mostly because my understanding is that RCS is meant to be a drop-in replacement for SMS and if you’re on a device that supports it (or your carrier-specific configuration of RCS) you don’t actually have a choice and your “SMS” is actually RCS and you must use it and hope it works.
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.
