Monopoly is the wrong term here, the root-cause of this situation is that Google is now maintaining the largest OS-implementation of RCS by far, only followed by Samsung, and then some carriers who also use RCS for other platforms (web-apps, STB,...).
Apple adopting RCS means that there is another player to balance this field. A much-needed player, and the only one that's relevant (other than maybe WeChat and WhatsApp)-
> With very few exceptions, it all goes through Google.
Yeah, because the mobile end-device for RCS is currently always an Android device. If there would be support from Apple, an RCS-server of Apple could handle the communication to Apple-Devices.
The idea of every carrier owning his own RCS-server was anyway already dead before the first client was released. The number of carriers never increased after the initial rollout of the major working-group members, simply because there is no way to charge for the service and monetization of such a small userbase with other services was not sustainable.
It's worth to put this into time-context here: When work on RCS (or "RCS-e") was initiated, WhatsApp was already well-established and leading the European market.