Well, from my experience seeing how people use Windows, it also comes down to the fact that the start menu was being used less and less.
That is what they say. However, seems tons of people are complaining about it..so that seems odd. In addition, I work in a corporate environment where nearly everybody uses their Start menu. Lots of power users utilize quick launch and other tricks, but the majority of just average users tend to use either the start menu to launch an app or they create a shortcut on their desktop (but they use the Start menu to find that application in the first place). These are the same type of people who use the Network icon to browse network shares instead of hitting the start menu and typing \\servername.
What they're offering now is a unified, fluid user experience where devices work better together.
Their hope is that everything works better together. Having a unified interface can certainly make things more consistent for people, but with the differences in devices and how people use them, one common interface may not be the best. For example, with desktop computers and such today without touch screen interfaces, I question the value of the Metro interface. Sure, it's consistent with the phone and the game console......but having a full screen application isn't ideal to many computer users.