RCS has been around since 2013 and the only alternative client to Google Messages was your device maker's app that was allowed to tie into Google Messages via a device vendor only API (but essentially these were just a skin for Google Messages).

Google has not provided any public API for RCS, and they control the ecosystem fully. If Google wanted other clients to use RCS as a platform, there would be public APIs that Signal, iMessage, Textra, Facebook Messenger and other SMS clients could integrate with.

Alright, thanks for the clarification. It looks like they opened it up for particular apps in the last few years but haven't yet gone beyond that.