Counterpoint: The next IMHO-blindingly-obvious step in computer evolution is that your tablet/cell phone becomes your desktop when plugged into a probably-USB-based dock that adds a keyboard, mouse, and real desktop monitor (or more). Tablets that can't run real OSes days are numbered.
Having a well-delineated tablet UI layer is good. We've got over a decade of proof that just smashing a conventional desktop onto a tablet doesn't work. But I would not want to massively commit to the idea that tablets will never run Excel. They will... just not in their touch-only mode.
It's all about how committed you end up to it, though; in the short term, if only Metro is on the tablet of 2012, that's OK, as long as you can pivot, potentially as soon as early 2013. (Prototypes of this ought to exist somewhere now if MS is going to pull this off.) But if you've written this requirement deep into the tech stack, you're in trouble.
Thanks for publicly mentioning that (seriously): Your mention in itself is now "prior art" for pretty much anyone who might later try to patent the idea. (IIRC, Arthur C. Clarke described something much like that in one of his Rama novels back in the late 1970s or early 1980s.)
The idea is obvious, yes, but USB? Thunderbolt is, IMO, much more likely to be _the_ connector. It has the speed to do the video that USB (even USB 3) lacks (power requirements may be a problem, but I think USB 3 has the same problem)
I also think that the parties building those docks will want to include a 'cloud connection', for which they will charge whatever they can, because, in general, hardware is not a business to make money in anymore. I am not sure whether they will get away with that. One thing that will depend on is what the economy will be doing.
If iOS apps could exist as say, dashboard widgets, there would be no difference (Apple probably thinks this is a silly and sales-reducing idea).
EDIT: Changed Windows 2000 to Windows Me. Windows 2000 was generally considered to be a successful release.
Apple also seems to be headed in the same direction with its introduction iOS features into Lion: http://blog.byjoemoon.com/post/3556631202/touch-ui-is-not-th...