There is exactly one app that can use rcs. No better than apple
Confidently incorrect but ok.
The whole point of RCS is that it's a protocol so there demonstrably would be other apps. It's like saying there's only one app for SMS, it isn't true.
What non Google sms app that has implemented RCS?
RCS seems to be pretty openly licensed out to other OEMs, definitely a lot better than iMessage.
It's still proprietary though, a far cry from something like Matrix.
Then everyone needs to download and make an account on said app, they already exist and none are unanimous.
Everyone has a phone number that gets used for auth and other things. If that system doesn't change then RCS is way better.