Kind of depends where you get your stats from, according to the site that I use it's gone down by almost 13% (and I'm sure will continue to drop) in the last 12 months (sept 2011 - sept 2012) from 40.59% down to only 27.64% I've seen some people complain about lag in the game and they're using a more modern computer and a more modern OS, so it would only make me wonder just how many of those 27.64% of XP users would have the hardware necessary to even run the game at an acceptable performance level if it were even possible to run the game on an XP OS.
In any event, I applaud developers of games for moving forward and at least trying to keep up with the times. I do remember a time that they were seemingly stuck in a slump and not even trying to take advantage of newer technology. Of course that feeling comes from somebody that has been lucky enough to be able to update my computers every year or so. I fully understand that not everybody is in that position.