My problem with this is that it is "Google Messages end-to-end encryption", rather than "RCS end-to-end encryption". It's not a distinction without a difference because it ties me down to a single RCS client and discourages interoperability. The moment I send an RCS message over Google Messages to someone who is using a different RCS app, I'm assuming it will just send plaintext.

Having Google Messages preinstalled on all Android phones and set as the default is not a good solution either. Encryption has to be baked into the protocol, not individual clients.