In the end just because Microsoft needs the desktop around for guys like us it doesn't mean that much of the rest of the world can't shift over to the Windows 8 Store App environment. That will leave Microsoft with little incentive to do much with the desktop. It all comes down to what's going to generate revenue...
But, also the "guys like us" do exist within Microsoft developing the tools and the software to support that vision. For this very fact, I don't see the desktop fading away soon.
Regardless, if you assume that metro supported dev environments (that possibly transcend device boundaries) are impossible to use for serious development purposes, then dev teams for Windows, etc.. probably would lean towards keeping the desktop environment as much as possible. On the other hand, if you allow for metro-like dev tools to actually work well for dev purposes, then it shouldn't really be an issue if the desktop stayed or didn't.