More fundamentally, the OP assumes that the rise of smartphones (and, it assumes, tablets, but I think that remains to be proven) constitutes a shift
away from the desktop platform.
There's no reason to believe that this is true, at least for most of MS's customers.
EDIT: after a bit more reflection, I think that while I can imagine a future in which everything runs off mobile devices, that is not possible with the mobile platforms that we have today. Further evolution of the platform would be necessary -- and that could well involve a shift away from today's current leaders (Apple, Google) toward the providers who already know how to provide the computing horsepower that business users and other tasks (PhotoShop, etc.) require.