1. Requires data connection, either WiFi or mobile data. SMS does not.
2. On Android, it goes through Google Messages. I don't trust Google.
So, I replaced Messages with another SMS app that doesn't spy on me and disabled RCS. If I wanted to do rich messaging through data connection, I would use Signal or similar.
So, RCS is not a replacement for SMS. It's just another IM protocol. And we have enough of those.
As for others replying through RCS, that's a failure of the messaging app, should recognize that the counterpart is SMS and reply in SMS. It's an attempt to force RCS...
SMS is for fast short messaging between mobile users, and that is its killer feature, it's universal. Every mobile phone supports it.
Signal dropping SMS support makes no difference for me, never used it for SMS, I used Signal for the secure conversations. Two different methods of contact.
I think you know this based on the rest of your post but there is no “another SMS app that doesn’t spy on me”.