They bought an implementation of RCS, the standard is still open. They even sell there implementation to a few telcos.

You cant self host it because you need to be a telco, it has telco specific protocols. Also, this is one of the things that probably should be gatekept to telcos, if everyone were able to spin up their own instance, the spam problem would be unmanageable.

Your looking for a conspiracy/monopoly that for once, doesnt exist due to Google being malicious. It exists because telcos are shit.

Google didn't buy it, it's an open standard.

What google did is host RCS infrastructure and have the worlds most popular messaging client.

You can absolutely setup an RCS server, build an FCS client that has the appropriate keys and access, put RCS relays in important peering points a over the world, and roll that out to millions of ppl yourself.

Sort of like nuclear power; you could probably build a reactor yourself, but it's a heavy lift plus there are players that don't want you to.