I really don't see Win8 as being the killer of the keyboard and mouse. My keyboard and mouse work just fine even with Metro. What I do see it representing as a shift in the way people think about the way they use their computers. To me, the last time Windows changed this much was from 3.11 to 95. Even the jump from XP to Vista/7 wasn't this large in terms of UI. Is this a good thing? Maybe, maybe not. Some people won't like the change, especially initially. A lot of people still don't like the look of the Office Ribbon, but many have grown to understand it and actually use it better than the old menus.
The problem I see with having the "classic" mode included is that you'll never be able to get rid of it. There was actually a huge uproar from the "old school" community when the classic start menu was removed from Windows Vista (or was it 7?) There will never be a good point to take out the legacy components, because someone will always say it's a bad idea. If it never happened, we'd still have the option of using a file manager sitting inside a program manager container and there wouldn't be a desktop.
Like I said, the keyboard and mouse aren't going away any time soon. Win8 is an attempt to blend the new tactile interfaces with the old. Maybe it's a little weird to most people at first, but give it some time. The touch-esque interfaces aren't going away and they'll be with us from here on out. At the same time, the keyboard and mouse are here to stay and will always continue to function. One of the best things about Microsoft OSes is that they don't really obsolete any existing technology, there's always though on how to integrate legacy components and keep them working with new releases. The people in Redmond aren't trying to do something drastic like kill off a processor architecture or force everyone to give up floppy disks and legacy ports. For that, you'd have to look at the folks in Cupertino.
I'd be curious to know though... What scenarios have you run into that don't work on Win8 (aside from not having the old start menu)?