To be honest, I think they might find after it's turned on that the limitation to the Google Messages app (which is not the default on Samsung devices and hence represents very little in terms of market share) doesn't move the needle at all and makes very little difference. Certainly, the impact of turning it on on the UK market seems to have been zero.
If Google follows through by enabling it system wide in future OS versions that might change a little, but Android being as it is that will take a long, long time, and the carriers probably suspect their own RCS implementations will have worked before then, or Whatsapp will just take over the US too and RCS will be dead.
I do use the Google Messages app, but I don't plan to enable RCS any time soon. The main reason is that I use the "SMS Backup & Restore" app to back up my SMSes & MMSes, and it's unable to back up RCS messages because they're stored in a separate DB that currently has no system API access.
So chalk that up as perhaps another reason why some people won't "upgrade".
I doubt Whatsapp will take over US too, the network effect is probably too strong for that to happen. For the same reason FB Messenger seems to still dominate some markets and Telegram others, even if theoretically Whatsapp could take both over because neighbouring countries are using them.
If Google follows through by enabling it system wide in future OS versions that might change a little, but Android being as it is that will take a long, long time, and the carriers probably suspect their own RCS implementations will have worked before then, or Whatsapp will just take over the US too and RCS will be dead.