They've invested so much money into their new app models and cloud infrastructure that completely abandoning the operating system that integrated those new services doesn't make sense to me. I can see them giving you the option when you install Windows to select the experience you want. They'll probably make it so you can change your experience at any time in the Control Panel.

The Metro UI design has been successful, imo. All you have to do is examine the changes to user interfaces and you will notice many are adopting the same concepts of Metro UI. Gmail now has a very flat look that isn't much different from outlook.com. Apple now uses flat colors and larger icons. If the design was a failure then Google and Apple wouldn't be copying it. Design is just one aspect of user interface and user experience.