You say that like it's a bad thing ("get away"). iMessage has nowhere near a majority and Apple doesn't put in any restrictions against alternative messaging software (…not that they're perfect, and haven't in other areas…).

I don't believe in closed protocols or crappy interoperability. There are several approaches that could improve things, like adopting Google's encryption improvements to RCS so that mixed iPhone/Android conversations are secure ("but that's not in the standard!", well, then, get it or something similar in the standard); they don't have to let others into iMessage necessarily. Apple claims to care strongly about their users' privacy and correctly attacks Google for caring a whole lot less. Encrypted, full-featured messaging would benefit their own users.