What you refer to as "Metro" fills a useful function that isn't otherwise served on Windows, which is enabling touch screen use,
Except that I've used the touch versions of XP, Vista and 7 and they all worked fine without it. Actually they worked better, because your finger worked like your mouse which meant that the same UI worked for everything in exactly the same way, once you figured out the conventions for left and right click, and naturally you need to make some elements a bit larger for fat fingers to click on (which works well for mouse users on high res screens and mouse users with poor motor control).
But that's actually beside the point.
What I was getting at was really that Windows 8 is a marketplace disaster. If, as a result of that if microsoft completely or largely scraps the underlying technology that powers modern apps (the Metro language) then we're going to be starting from the beginning. Now they don't have to do that necessarily, but they might need or want to do significant rewrites of major pieces of functionality to try and make it their version of better for windows 9.
If you want to make the case that keyboard and mouse users shouldn't have to look at the Modern UI if they don't want to... why then I will heartily agree.
See this is the problem. Modern UI apps, that run in a window - which is something we might see patched into Windows 8.1.x or 8.2 or something, and is available from aftermarket stuff (from Stardock I think), already does that, and then they work, well, basically fine in that it's just another design language from microsoft and if you use it well it works well, and if you use it badly it works badly. And you don't need or want to block of desktop users and mobile/touch users. It's basically extending the gadget concept, and in general it's the sort of thing that makes a lot of sense on a second screen, a lot of small pieces of live updated information that tells you about all the stuff that isn't your immediate focus (the main screen) but that you can, at a glance, get an overview of many things all at once.