Contrary to popular belief, Metro is not a replacement for the Start Menu. Metro is a replacement for the Explorer Shell. The Explorer Shell itself has been turned into an application. Traditional applications run within this Explorer Shell, and cannot be managed from Metro. In other words, the Explorer Shell has become an application with a multiple document interface, running in Metro.
This, right here, is the main reason why Windows 8 is such a pain to use with a mouse and keyboard. You can't directly switch to a desktop application; you always have to first switch to the Explorer Shell, and then switch to the desktop application you want running within the Explorer Shell. This is a convoluted way of using my computer, especially since Metro itself isn't mouse-friendly to begin with, with finicky hot corners and UI elements that are too volatile.
Consider this. To switch to a Chrome browser tab, you have to: switch to the Explorer Shell in the Metro application switcher (and hope this doesn't go wrong), switch to Chrome in the traditional taskbar, and then switch to the right tab within Chrome. This is insanity. Whenever I go through this in Windows 8, I hear this playing in my head.
It's not a technical issue. Microsoft could easily integrate the two much more efficiently and more fluently if they wanted to. No, the real issue is that Microsoft doesn't want to, because (and here's the pill that's so tough for some to swallow) the Explorer Shell is being deprecated. It's dead. It needs to be cumbersome and unpleasant because Microsoft hopes this will make users demand Metro versions of their favourite applications.
Trying to shoehorn a tablet/smartphone user interface paradigm into a desktop/laptop computer is just as flawed as trying to do it the other way around. Unless Microsoft has some sort of grand plan to reintroduce proper window management into Metro, I don't see how I could ever get any serious form of work done with it.
I have the sneaking suspicion Windows 7's going to be around for a while.