Why does everyone think this is unlikely rather than inevitable? We already know not only that they can get away with it successfully, but their staff doesn't even feel it's an unethical way to treat their customers or the industry.
They were up on stage yesterday talking up the new AutoCAD port. The new AutoCAD port which is utterly incompatible with the posted App Store rules. The AutoCAD port that would in no way benefit from the exposure the App Store would give it, and which doesn't need and would never accept the 30% sales overhead for payment processing.
Locking the platform to the App Store would be a great way to kill Office, MatLab, Photoshop, Lightroom, AutoCAD, Steam, and a bunch of other software the platform needs if they want to keep anyone using it. To assume that it is inevitable that Apple will do it is to assume that Apple has a death-wish for the Mac platform.
But if we're going to assume that, why not just assume they'd kill the platform in the most straight-forward way possible: by discontinuing it.
Besides, imagine the marketing pitch for that version of OSX. "Upgrade to our latest version! Most of your old software will stop working, and you'll have to buy it again!" I don't think so.
They're going to make a lot of money with their app store -- they don't need to destroy the rest of their platform to do it, though.
Complaining that the App Store will not carry all available MacOS Apps is almost as silly as complaining that the Apple Store at your local mall does not carry all available MacOS Apps. (`almost' because the shelf space of an online store is, presumably, less limited.)
Now, if there were a move to make the App Store the only way to get (or even the only way to buy) apps for the Mac, that would be different. But as far as I can tell, there isn't.