A long thread so these points may have been made already. Apologies if they have.
Firstly, anyone - including Apple - who thinks someone will buy an iPhone because of iMessage is deluding themselves. No one will buy a £700 piece of hardware for a messaging app that can only be used to its full potential with other iDevice users. Even the on-contract pricing won't make much difference to this. I agree with what others have said in that keeping iMessage exclusive to iOS could stop people defecting to Android, but I don't believe for a minute anyone would switch the other way just for iMessage.
Secondly, there is a way to monetise iMessage although admittedly not massively. The iMessage App Store. I don't believe for a minute that the same greedy devs who scrape people's wallets for gold and lives via in app purchases won't launch "pay per use" stickers. Personally I'm dreading that possibility but I can almost guarantee it will come.
I think they should go cross platform with iMessage or drop it entirely and let FB Messenger, Whatsapp, and their like have the market space. They're cross platform, do most of the same stuff, and are already being used by millions. Yes they may lose some customers to Android in that case, but I can't see them gaining customers just for iMessage.
Having said that, I doubt sticking iMessage on Android will entice people to use it instead of their current favourite messaging app anyway.