Not to mention the technical difficulties. How would they stop me from running an application which I'd copied off a CD, or compiled myself, when I already have control over my filesystem? I suppose they could figure out a way, but it'd involve a lot of voodoo.

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.