Google could encrypt the content with end-to-end encryption like Moxie did with TextSecure and SMS. The RCS content would be encrypted between Android Chat users.
But from every move Google has made recently, it seems to think that people have forgotten about the Snowden stories. So they now have ZERO interest in anything that is end-to-end encrypted, because they think they can now get away with it.
They threw an option in there for Allo, but it was never meant to be used by users anyway, and now they're killing Allo, too. I don't think they'll ever talk about end-to-end encryption again because they want all of that data for themselves.
Now, the very least Google could do for its users is encrypt the RCS messages between its Android Chat users with some kind of Noise/HTTPS encryption. That would still give them all the user data, but at least they'd be protecting users against the carriers exploiting all of that data, too. So it would be very similar to them using HTTPS for websites or for DNS, and I see no reason why Google wouldn't do this.
I still wouldn't use Android Chat because I don't trust Google at all anymore, especially after their recent interest in working with the Pentagon for developing autonomous military drones, but at least it would benefit everyone else.
Also, there was a recent thread on /r/Android, and it seems nobody outside of US cares about SMS anymore (which means they don't care about RCS either, which is basically SMS+):
https://www.reddit.com/r/Android/comments/8dvm9x/nonus_based...