The Desktop is at its final evolution.
The desktop is at its last iteration. I personally believe there is nothing more Microsoft can do to improve it (UI wise). I also believe that the desktop will always be there for those who need it. Just like the command line, the desktop will, in the far future, become niche and obscure.
Microsoft has no business trying to combine the metro world and the desktop world. They can work together and compliment each other, but they will not be combined.
Now, metro is still in its early day. We will still have to go back to the desktop for some things, but as Microsoft develops metro more and apps get stronger api(s), we will see less need to comeback to the desktop.