If I'm using a computer and someone sends me something on iMessage, I just won't get it. If someone wants to start a real group conversation on iMessage, they will need to download an app because it lacks that capability. If someone wants to start a video chat with me on iMessage, they will need an app because it lacks that capability. Sure, they can send a text message to a person that doesn't have an iPhone, but that's been possible for a couple decades and truly is nothing special nor worth touting. iMessage is extremely vanilla and isn't very useful except under very specific circumstances (where both users have an iPhone), and even then there isn't very much benefit to it. Babel is a completely different animal.