The fact of the matter remains when you remove all the 'personal' feelings from the issue is a simple one and one other caveat.
Can MS build a system that can be maintained and still work across many interfaces and devices?
Can or will the end-users be able to easily navigate and switch operational modes?
The first is imperative in terms of support. One set of source code, one fix, albeit, there could be many branches in the code to support different systems. Single source allows fixes to be made to all versions easily.
The last one is really end-user training and resistance. Are users 'willing' to switch how they operate. Can MS make the look and feel be seamless between mouse/keyboard, touch screens, and mobile devices, both phone and tablets?
One major change now is 'no close button'. You just don't end tasks/programs. You 'switch away' from it and the OS will 'kill' it when need be. Are the millions of mouse/keyboard users going to 'like' this and easily adapt? Navigation IS different, but I'm confident 3rd party developers will 'fix' that.
Of more importance are the 'new things', like an apps store. Also will W8 create new h/w sales where touch screens become the norm?
Lastly, what is it IN W8 that will make people to want to switch? Or will this be 'forced' on you when you buy new computers?
Think of Apple though... they have computers and phones and tablets. Each sort of works different, or I should say the desktop/laptops vs. phones and tablets. No forcing one on the others. However, Apple does have to support more than one OS. Who is right here, Apple or MS, and don't forget the other fringe players today as well.
This is all about 'dominance' and 'where the money is', nothing else.