1) Windows 8 is changing Windows in pretty fundamental ways (being touch first, leaving the "PC" behind), and I can't foresee anything but minor changes in Windows 9 to this direction. I doubt they will try to go back to making Windows 9 a true successor of Windows 7. That seems very unlikely to me now. Microsoft is all-in with this tile-based version of Windows. Even their logo has changed to reflect that. So businesses who don't want to stay on Windows 7 forever, should take into account alternatives.
2) Even if you hated Windows Vista, there was nowhere to turn in 2005 but Windows XP. Now there are some pretty good alternatives, and people are getting used to using different operating systems than Windows, which I think is a huge deal, because it's usually very hard to convince users to use another OS.