Apple may not be exactly jumping in joy about it but, even if it's mostly only useful for people in the US, they probably don't see that as a showstopper either.
And the alternative on iOS is...? What exactly?
RCS is a flaming trash dump of failure.
Other messaging platforms; the one most commonly cited seems to be WhatsApp.
On iOS? Don't make me laugh. Apple will die before they implement custom configurable messaging providers in iOS.