Also, it is just fake open-source, at the end of the day all your data goes through google’s servers, with optional encryption that will be decrypted when you talk to an old device — that’s security theater.

Doesn't the encryption support depend on the messaging app? Meaning old devices could easily just update (even auto-update) to a newer version of Messages that does support encryption?

Google did not create RCS. Google has endorsed RCS and implemented it to some extent.