It’s RCS not RMS and Google didnt even want control of it in the first place, it’s well documented Google has been trying to get US carriers to stop dragging their feet on RCS for a long time. They never did until Google literally went “Fine, I’ll do it myself then”
AND RCS is an open protocol, nobody really has “control” over it, Google runs some RCS servers but if it disappeared tomorrow (Or you changed the defaults) RCS itself would run just fine on whatever including if Apple supports RCS
ETA: Also SMS is absolute trash, it’s from the early 90’s (it’s older than me FFS) it doesn’t really support what we want out of it media wise today, and what it does support it was forced to. It’ll send “video” but it’ll be completely unrecognizable. It needs to be put to pasture already.