I don't think Windows was ever "popular".
Keep in mind that Windows' outrageous market share wasn't because Windows was "popular", or "better", it was because it came preinstalled with almost all computer systems.
It was Microsoft's marketing and distribution strategies that made it so widespread. And no, it wasn't solely Bill Gates. Don't forget about crazy Steve Ballmer who runs the company day-to-day. DEVELOPERS DEVELOPERS DEVELOPERS DEVELOPERS
I would say Windows is more ubiquitous than popular. It found it's way onto the majority of our computers by placement, not choice. The majority of Windows users don't even understand the concept of an operating system, let alone the reality of several different options and alternatives.
Most people won't migrate to any new OS anyway, because they're too comfortable with what they already have. They're just not ready to take the leap and "learn" something unfamiliar. It was a big hill to climb learning how to work their Windows boxes, and they don't want to do it again.
I believe this is the very principle that is plaguing the adoption of Mac OS X and desktop Linux distros. They don't want to re-learn how to work a computer. To add insult to injury, I believe these two OS's are lightyears ahead of Windows in terms of user-friendliness and intuitiveness. The exact kind of user-friendliness and intuitiveness that's keeping them committed to Windows.
True, the market has a history of revolving around the Windows platform, but this is only a byproduct of business. They will only invest the resources into the platform with the largest user-base; undoubtedly Windows. But with the steady increase in Mac and Linux adoption, the market could shift towards a more diverse strategy. As user-bases begin to even out, so will the market's extreme bias towards all things Microsoft.
All great things must come to an end. The next big thing is still on the horizon. The once-unstoppable IBM no longer produces PCs. The almighty Yahoo!, once the de facto standard for search engines, is now only one of several options. Even the wildly popular Myspace has recently been surpassed by Facebook.
Windows has, without question, finally plateaued. It's only a matter of time before we will all witness it's fall from grace.