Having suffered the Vista backlash, I just can't understand why MS would cut their own throat by not allowing people to remove such an obvious barrier to sales, and allow laptop/desktop buyers to choose their default UI - Metro or Classic. The only thing I can put it down to, is some huge egos getting in the way.
Wouldn't experience with the Vista backlash (and Win7 success) actually reinforce that rolling back to the previous version (or providing an option to do so) isn't necessarily the way to approach situations like this? Win7 mostly didn't roll anything back from Vista to XP. It kept the Vista start menu, Vista Explorer, Vista Control Panel, Vista visual theme, Vista driver and security models etc. The changes it did make, like the taskbar, were about moving forward to something new, not rolling back to XP.
Even if there is a backlash to Win8 I would hope that Win9 is about solving whatever problems people have in new ways, not rolling back to the Win7 approach.