I see what you did there.
As soon as Ubuntu disallows non Ubuntu designed software from running, you might see some consistency. But what would the reaction in the linux community be to such a move?
They do not have to disallow anything to make Ubuntu more consistent: Apple does not disallow anything from being installed on OS X (even now that the Mac App Store exists) and yet OS X is a lot more consistent than any currently existing Linux distro.
I don't think you can compare Mac OSX to Ubuntu in this way. The mac developer culture is different from the linux developer culture, due to the strong presence of the original UI design and human interface guidelines. When something doesn't match the OSX experience, its easy to notice- because you have that strong baseline, and there's a strong motivation to write software that matches that baseline experience. software that runs in OSX's x11 is obviously not as good as "real" os x software. There is no base for linux, so any widget set or toolkit is just as good as another, and there's no strong reason to pick one or another language, or "platform" or library. You're invited to install this runtime or that scripting language to get arbitrary programs to run. And linux people like it that way. That's what freedom looks like.
Ubuntu has already been flamed to a crisp over Unity, so they might as well go all the way and remove all non-Unity apps. Fedora aka GNOME OS may beat them to it, though.
As hollerith said, Canonical just needs to create a strong default desktop experience such that everything the normal user wants fits into The Ubuntu Way. Weirdos like me will still be allowed to install Ubuntu, put Window Maker on it, and happily ignore The Ubuntu Way even as we take advantage of driver support and large and fairly up-to-date package repos.
