Ignoring some things I would consider flaws (lack of end-to-end encryption right now), from what I’ve read it sounds like the rollout of RCS has been something between a mess and a disaster if your friends are on another carrier.
It sounded like it got to the point where phone companies were delaying so long (at least in the US) that Google is basically forcing their hand by doing everything themselves if the carriers don’t.
Meanwhile iMessage has been running for 10 years.
That said I agree with you that I see basically no reason for Apple to add it. That’s up there with iMessage for Android in probability.