Hopefully a few hundred million XP users will upgrade. For me, it's all about Internet Explorer. If Win8 does well, IE10 will be the new minimum for html5, and it's a lot better than the previous IE's.
http://html5test.com/results/desktop.html
Most non-corporate Win7 users should get the IE10 update too. In a year IE8 usage will be in the single digits.