MMS is free at this point, in many countries. The carriers in those countries will make more money by using RCS, because it will use cellular data (at least if I understand it correctly), which isn't free.

My point being that I don't think many carriers care about text messaging, or phone calls. They sell you a fixed cost plan for those. The only thing that can really affect your price is data usage. If Google wants to deal with the hassle of managing a messaging platform, great, that's money save on running a service that isn't making money anyway.