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RCS is an open standard, anyone can implement it. Google just implements one such app. (Google also implements, uh... android... so bad news if you're trying to avoid Google and use an android)
If you're still using SMS or MMS, it's all unencrypted, always.
RCS is supported by /carriers/, and many do support it. It is open in that sense.
I don't think Google is "disallowing" others from implementing RCS, it's just others choose not to? Like, RCS/SMS is a core feature of the phone/os, so it doesn't really make sense for anyone other than the OEM (or Android) to care about it. Samsung is an OEM, and Google is Android.
Telegram e.g. isn't going to implement RCS becuase why would they? Just use telegram's protocol. Etc.
And carriers don't implement any apps today? When was the last time you had to install a carrier specific SMS app? It's always down to the OEM.