RCS is basically email over HTTP, wrapped in a layer of carrier stuff. The same way Visual Voicemail is IMAP but wrapped in a layer of carrier stuff.

The spec also handles video calls, conference calls, sending/receiving money, and just about anything else a modern messenger does.

It just lacked E2EE for the longest time, which makes sense when you consider that the police and secret service have their tendrils in the standards body that publishes the spec.