Did people call RCS open source? I'm not a huge follower of the standard, but I don't think I ever heard that said. In fact, I've heard people complain about not just the proprietary encryption but lack of E2E and carrier/Google control.
Its only advantages are that it is better than SMS and supported by the carriers, Google and Apple sometime this year.
It's a shitty standard but given how shitty SMS is, I'm willing to hold my nose and jump in.