RCS is not completely compatible. For one thing, it requires a carrier and a phone number. You can go out today, buy a Mac or an iPad with no cell modem, and start using iMessage purely as an IP messaging app. So they can't just replace the existing protocol with RCS, because RCS is a bag of flaming shit. They could spend the money to develop RCS fallback in addition to their protocol, and that would be awesome, but it costs them money, and I get why they don't want to do it.

The reality is that this is Google's fault more than anything. They spent half of my adult life repeatedly inventing and then fucking up the act of sending 200 bytes of text to one person at a time.

I'd love for the modern world to have a great way of messaging people that just worked -- used IP connections with SMS fallback, a login you could manage from anywhere, full support for all the real-time typing stuff, the rich media support, the whole thing. That would be great. Someone get on that. But if I have to listen to fucking Google whine about it one more time, I'm out. They're like a guy with one finger left. If you didn't know any better, you'd feel pretty sympathetic for him. But if you've spent the past two watching him slowly chop the other nine off one at a time with a hatchet and then whine about his bad luck for 12 hours after each chop, the sympathy starts to ebb a bit.