Most businesses, consumers, and developers universally continue to ignore the primary reason that iMessage is a closed platform, rather than an app on every platform as iTunes is: Apple is using device serial numbers for anti-spam, supported by a fully-authenticated hardware and software stack that does not allow user modification. This permits Apple to simply “console ban” any Apple device that spams on iMessage. Th…
How does unmodified software relate in any way to the ability to console-ban bad actors? It's apple's servers, apple's accounts, and apple's devices. They are perfectly capable of burning a private key into the fuses of every device they sell, keeping a revocation list, and requiring a valid signature from an unrevoked key to log in and send messages. You can't get around that with any quantity of homebrew or custom software. Same reason that you don't see spam on Nintendo Switch games - if Nintendo bans your hardware you're not getting back online unless you buy a new Switch, and that's enough of a cost to make spam uneconomical. You can't do that with Android because maintaining a single revocation list across many manufacturers would be impossible - or because Google would have to host it and they'd get mobbed by angry HNers frothing at the mouth about their privacy - but Apple is totally capable of it and already gets a free pass on whatever walled garden shenanigans they can imagine.