It was probably easier to release a whole new paradigm with [whatever Metro is supposed to be called], than fix all the bugs with MFC and maintain backwards compatibility. As a side note, whoever came up with the licensing scheme for the ribbon interface, really screwed the pooch. If Microsoft gets around to fixing the desktop UI story, it will have the ribbon as a main component.
If you look at all the new APIs in Windows8.x, there aren't many that are dependent on the new UI, even if you are forced to use it, to use them. The way I figure it, the new UI was supposed to be a lead-in to phone development, because there are going to be billions of phones out there, but never billions of computers.
It's relatively easy to make a phone every couple of years that makes the old model look... old. Compare that to the PC. You would be hard pressed to tell the difference between a 2014 PC and a 2011 PC, but a 2011 phone would immediately appear antiquated to just about anyone.