You're ignoring key differences in how MS desktop and mobile products have been marketed (both in the past and present) and how they will be marketed in the future.

Til now, they have been stand-alone products. WinCE/WP7 had little in common with Win desktop - not the OS, nor could software crossover.

With Win8 that changes. There will be an app store common to mobile and desktop systems. One-stop shopping for all MS users on all MS systems. As billions of MS desktops update to Win8 and newer, this will create a huge advantage Linux, Android and even Apple will not be able to match. Even if most ordinary users could be convinced to switch OS, business lock-in to MS systems guarantee MS a huge market share (and income) for many years to come.
Not to mention the effect that unification of mobile and desktop systems will have on developers...

M$ is far from done. I just hope I'm 100% wrong.