Metro is nice, but it's nice on a tablet. I wouldn't want to use it as a development or business platform. Microsoft believes this as well. Where's metro visual studio? Where's metro Office? Even on an RT, Office is on the desktop. What about gaming? NVidia and AMD rely on Windows to make the OS that their enthusiasts games run on. There are no demanding metro games. Hydro Thunder? Pft. Even the metro version of 3Dmark is a joke. If they were to try to kill the desktop all they'd accomplish is a ton of people would refuse to upgrade, just like they did with 8, or they'd jump ship to OS X.
Which would be a shame. Windows 8 on the right device, like the Surface series, is a joy to use. It's amazing, and something I'd never have expected Microsoft to pull off as well as it has, but it's not complete without the desktop half. Maybe it can slide on an RT because it's a tablet, and you probably already have a desktop computer, but if you were to go to the store and buy a desktop, and it only has metro apps, and no desktop, it'd be a joke.