But they already do support Android to some extent. Apple Music (don't know if you can subscribe via the Android app), Shazam, and an AirTag detector are all already available.

The point is “are they forced to do so”

It seems weird to degrade their own users' experience (when receiving texts from friends with Android phones), but Apple does it deliberately as a nudge to get people to use Apple products.

There's no valid technical or security reason to do this. It's a tactical decision on Apple's part.

If iMessages have benefits (they do) then there is a technical reason to show you the bubble colors - so you know the benefits apply. If sending video to a blue contact is better than sending it to green, there's a reason to know.

Does it ACTUALLY matter? Maybe not? But people really do complain about a non-iPhone "degrading" a group chat, so it is indicating something.

At the time they made iMessage at first? It was likely a real advance and only because they could control both ends. But now? They may be large enough that it's unfair use of their monopoly in one area to affect another, and get slapped or forced to interoperate.

Why not both?

The argument that the color of a message bubble is tantamount to a "degraded experience" is truly bizarre.

Or how about if, I don't know, one of them you have to pay your carrier for, and one of them is free?

That might be worth letting the user know about it.

There are many industries in the world regulated to be interoperable. I suspect the primary reason you find the notion weird is simply because you're not used to it.