Writing is on the wall, so to speak -- MS is fairly predictable over the decades; introducing something new, and dropping support for something old. Given Win RT (the ARM branch) is totally locked down, and theres a big push for the locked down store on Win8 desktop ... its akin to MSDOS and Win3.x etc .. The traditional desktop 'do as you want' is on the way out, they've got plans but aren't quite suicidal enough to hang their existing customers yet.
I imagine they'll cut to closed shop (since they want to be Apple) in a couple years, and keep an Enterprise side-install option.. of course, its all guess work.
but do _you_ want to base your company arond guesswork? Surely Gabe cannot, so he has to plan.
(FWIW, recall that Valve has enjoyed many yers as essentially the defacto game store for Windows.. but as a consumer, its at least a little better to have an independant doing it, than Microsoft. But really, with Apple and MS both doing walled gardens, and Android half way there.. its an ugly spot to be in for consumers..)
/me sighs
jeff