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.