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.
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.