While Android users might appreciate having access to iMessage, the bigger benefit would be for iPhone users, who’d never again have to suffer SMS’s limitations.
I’m not sure if people realise what a big pull iMessage is. Users that would never consider switching may not realise that iMessage is one of the big players in making people stay on iOS.
Those that are exposed to iMessage for even a small amount of time know what a pain it is to have to go back to regular SMS. Google can try and fix it all it likes but regular SMS is horrible.
This is without considering the fact that I don’t think Apple could convince carriers to get on board. If they tried to do something to iMessage now I think the carriers would resist it.