RCS is not a slam dunk. Juniper Research says RCS-capable subscribers represent less than 13% of global mobile subscribers in 2022. (They project that that will double to 40% by 2026, FWIW.)
End-to-encryption is not part of the RCS standard, but is instead a Google-proprietary layer on top of that. I assume that Apple can't seriously consider adopting RCS until that omission is fixed. It would be crazy for Apple to create a Google dependency for something as critical as messaging, especially given Google's long history of failures in this area.
