>You're not being serious are you? MAS holds such an insignificant role in Mac App development that any requirements it sets around the transition would weaken its cause.

Negligible? If you exclude Adobe and MS, most apps people use are available through the Mac App store. And more can be made available if Apple pushes more for it.

Hypothetically, how do you think the market would respond if Apple said that apps could only be sold through the store?

With the restrictions it currently imposes (sandboxing), I think we would see a significant withdrawal from the Mac platform. Many developers would stop making apps (especially those that are for power users, which when turn more devs away from macs). I doubt Adobe Creative Suite (Photoshop) would be technically possible with sandboxing, so Adobe would withdraw from Mac. MS Office would probably be gone as well. Parallels and other virtualisation software would find it extreme difficult to run.

Valve would have to pull Steam because you can be as sure as hell they aren't going to give Apple a 30% cut of Steam sales. There goes Skype as well.

Without special deals with lots of developers, I doubt we would see many more apps on the App Store and instead we'll see the Mac be an unsuitable platform for many/most people. It would turn into a glorified Chromebook.

Except that all the real desktop alternatives are adopting the same sandbox models.

So, just like any technology adoption that comes from above those developers would just shut up and follow along.

Or do you think they would earn any money selling to GNU/Linux users?

In this hypothetical situation Microsoft doesnt make the stupid move of requiring all apps go through their store which imposes its own limitations (like sandboxing).

Mind you, it would be a completely insane move for Apple (or Microsoft) and I can't see them doing this any time soon. It would be absolute suicide.

Except there are no alternatives for those that sell software, as I mentioned on my comment FOSS users don't pay for desktop software.

There's all the ways people sell software on Microsoft platforms.

Adobe has managed to get thousands from users without any 'help' from Microsoft.

How is Adobe going to do it when Windows follows the same footsteps than Mac OS X?

The only way will be the store, or why do you think Microsoft is making it easy to port Win32/WPF (legacy) applications to the store model?

When the applications that matter like Adobe are on the store, and Apple has proven the "my way or the highway" works, they will slowly disable the "legacy" model.

I have been through enough computing changes to believe this will indeed happen.

Considering that the only app I use on my mac from the AppStore is xCode, I'd say it would respond very negatively.

If you use XCode you are already not the typical consumer.

True, but last time I checked Xcode is one of the most popular apps in the Mac App Store.

That says something