Some applications will work better in a touch screen environment, that's the aim of the Metro UI. Some applications, like Visual Studio, will work best within the traditional desktop environment. I think Microsoft is more about offering you a wide variety of choices on how to implement your application rather than forcing you to any single technology.

You don't have to rewrite your entire app. In fact, there are probably large parts of it you can reuse. But you can't expect a Win32-style UI to work in Metro, and you can't expect sandboxed apps downloaded from a marketplace to be running as admin and have full access to the registry, either.

Can WPF apps run on Win8? Yes. Are they preferred when the user is running an iPad-like device with no keyboard and fat-finger pointing accuracy? No. How about when the user has a keyboard, mouse, and desktop? Sure. Is Excel usable with fingers? Not really.

This is just like when Apple released OSX 10.5, and all of a sudden all the Carbon apps had to be rewritten. Except not; Microsoft will still let you write Win32 apps and run them on Windows 8, where Apple said "if you want to run on 64-bit OSX, rewrite."