You're right, I just googled (actually Kagi'd) it and found:

  With Google’s rollout of RCS as Android’s primary texting platform for Android Messages, many Android phones already come with Android Messages installed. A partnership between Google and Samsung allows RCS features to work seamlessly between the Samsung Messages and Android Messages apps, the default SMS apps on their devices. You can also go to the Google Play Store to download Messages yourself.

https://www.digitaltrends.com/mobile/what-is-rcs-messaging/

So that explains what happened, it still doesn't excuse forcing it, without permission or notice, on people who are using an open protocol

RCS is an open protocol, in fact it was developed with the GSM association https://en.wikipedia.org/wiki/Rich_Communication_Services

You seem to want every single reason to dislike anything Google has ever helped to make, rather than doing actual research into what some of these things are.

It's an open protocol but it's not an open system, just like SMS. You can't just set up your own RCS provider like you can with the web, email, IRC, etc. So it's at least a little misleading to call it "open".

The fact that it's an open protocol doesn't mean that it's okay to opt you into it without your permission.