Comment by TurningCanadian

iMessage is the monopoly part. They could make an App or even just an API available on other platforms but don't because they want the lock-in.

> “The #1 most difficult [reason] to leave the Apple universe app is iMessage ... iMessage amounts to serious lock-in,” was how one unnamed former Apple employee put it in an email in 2016 > “iMessage on Android would simply serve to remove [an] obstacle to iPhone families giving their kids Android phones,” https://www.theverge.com/2021/4/9/22375128/apple-imessage-an...

Not getting on board with RCS or any other way to improve SMS/MMS until they were (implicitly) forced was motivated by that desire to lock their users in to a messaging platform that only works on Apple devices.