XP/2000 is far better than Win9x. In many ways, including security and long-term stability. Vista, however, is not much of a change in any field except eye candy. I loath Windows, but I can't say XP is buggy nor completely insecure(Way better than 9x, but still has some general problems based on it's security model).
But there are still plenty of people using Win9x, which tells me that it will be a lot of years before XP/2000 dies. Gaming is the only reason I would upgrade(it's the only reason I use Windows at all). If I'm ever forced to upgrade to Vista, I'll run a dual boot with Vista/Linux, using Linux as my primary. By the time I'm forced into Vista, I'm certain Linux will be a polished OS ready for the consumer level.(it's damn close as it stands, just needs a few corners smoothed).
Not sure, maybe its a trend with this new generation of technology.
Because look at the ps3, I bet more ps2's are selling than ps3's still, and they are releasing more ps2 games still than they are ps3 games.