If this is true:

  It doesn't add another protocol, it ultimately replaces two of them.

How does this work (assuming your carrier supports MMS, and not all do):

  Everyone on RCS can fall back to MMS just fine

As for this:

  Even though Google Fi is definitely compatible with RCS

https://old.reddit.com/r/GoogleFi/comments/l1czwh/google_fi_...

More recently it looks like Google added some half assed support for RCS and broke other stuff in the process:

https://old.reddit.com/r/GoogleFi/comments/12b8k2p/reminder_...