In short, all the things Google or Microsoft are criticized for are Okay when Apple does it.
In short, any portable version would still be nerfed, just slightly less nerfed than the current situation. The green bubbles would still be there to indicate that their platform can't support core functionality of iOS inside of iMessage.
It's a marketing gimmick designed to lock users in using network effects.
These psychological tricks work the best on teenagers and apple have been targeting kids to make them customers for life.
I don't have a dog in this fight but no one complaining ever really addresses this. iMessage is not just a chat app, even though many people like to pretend it is. It only looks like a chat app if you are not on iOS.