And how will that fork get on any phones for customers to use? "People can just sideload the OS" is not a feasible answer here.

> "People can just sideload the OS" is not a feasible answer here.

Well, it's not feasible as long as bootloaders remain encrypted/non-unlockable. That's the problem with restricting the software that can run on hardware after the user has purchased the hardware and owns it.

People will just buy different Android phones from someone else.

We've already been through this. AT&T (uniquely among US carriers, I believe) blocked non-Market applications on all of their devices. The success of the Amazon Appstore forced them to change that policy.

Saying "the success" was what caused it isn't really accurate, though.

If it hadn't been for a large, US corporation lobbying AT&T to allow their product on the handsets, would it have gone through?

A future where AT&T allows the Amazon and Google app stores only is still one that could happen.

Only a part of Android is open source.