Saying "this is a better SMS app" got people on-board and let them "upgrade" to secure messaging. That's why I started using it in the "TextSecure" days.
But, sadly, I agree with Signal's reasoning here. Mixing the two protocols was annoyingly complex. If someone stopped using Signal, messages you sent to them would never arrive - with no notification. And there's no obvious way to "downgrade" to SMS.
I was working on RCS a decade ago. I'm glad to see it is finally getting somewhere - but I'm sad it is at the expense of better and more secure protocols.
I can imagine that's less of an issue in the US; do you pay extra for text messages and calls that go across state lines?
Fine by me. If they want a better experience, I have almost every messaging app on the Dutch market bridged to my Matrix server so I don't care, they'll just have to live with the lack of features if they want to chat with me (or install something like Signal).
I don't care about RCS and other ISP standards that exist to squeeze more money out of texting. I'll use Telegram/Signal/WhatsApp calling before I'll use my phone app because my subscription doesn't include free minutes (and I barely call anyway) so I've gotten the benefits of tech like WiFi calling through VoLTE for years before ISPs bothered sending their VoLTE profiles to my phone.