Your point basically boils down the whole issue of why Win8 is a failure: MS blindly followed Touch UI trends in a context where Touch UI is simply weaker than Mouse/Keyboard, and they threw away many of the advantages of a Mouse/Keyboard oriented UI in the process. Metro might have an advantage through its consistency across applications for Touch devices, but as long as it's being forced onto Desktop users it will continue to alienate a big chunk of Microsoft's core userbase. Considering the current developments I expect a full 180 for Win9 - even the Vista start menu might come back at one point.
I don't think touch and discoverability are mutually exclusive. I do think that discoverability is very difficult to do on 5" screens but that doesn't mean discoverability must be equally bad on a 10" or 20" or 24" screen.
Microsoft can't just do a u-turn and go back to the traditional desktop. What they need to do is to scrap the old desktop altogether and use the screen real estate that's there.
Why not implement a fully featured tiling window manager? On a small screen, you see only one or two tiles and you have no discoverability. On a larger screen you get extra tiles that show available commands and help text. Once you learned your shortcuts you can remove those tiles.
I like your idea of a new window manager. There are a few more aspects that need to be taken care of if one wants to bridge Touch and Desktop UIs IMO:
* contextual menus should still be available to speed up certain tasks. For touch screens these should be available through a simple gesture, for mouse/keyboard it's the right click as always. I'm thinking that for touch UI the menu entries could be placed in a circle around the point where the gesture has been initiated, without hiding anything behind the finger (the OS should recognize from what direction the finger has been placed on the screen).
* the concept of a program menu should be reimplemented. Every function should be accessible through this menu, even if there are contextual elements that implement them as well. However, this menu needs to be easily usable on touch screens as well.
* there's one element I'd propose to be always visible, no matter what screen size starting at 7 inch: The search. This search should be able to find application commands and data, OS commands, other applications as well as and system wide data (in that order).
