You don't know what you are talking about. I use RCS everyday and its encrypted. RCS has been encrypted by default since June of last year with group chat encryption rolling out soon.
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Google Messages end-to-end encryption for RCS Chat is rolling out to everyone
For one-on-one conversations, not group chatswww.androidpolice.com
That's only for Google's extension of RCS as far as I understand and not a feature of the standard RCS protocol itself.