I have a feeling these dropping "popularity" numbers are just a reflection of consumers getting fed up with being forced to pay more for an OS than their entire computer is worth. With the prices of decent computers dropping every year, Microsoft can't keep charging more and more for an OS. It's not like today's OS does anything revolutionary compared to an OS from 10-15 years ago. All they did was move some buttons around, plug some security holes, and fix some long-term safety issues.
Frankly, Windows 7 still lets you do precisely the same functions that Win 95 or even Win 3.1 did. You can access hard drives, printers, scanners, the Internet, play games, etc., just like you always could. Taking over a decade to change the GUI and fix some engineering flaws in your product doesn't make it ok to find a shady way to charge twice as much for the complete OS. Breaking the price into two pieces where you buy a crippled OS and then upgrade it to get the rest doesn't make the price any lower. No desktop OS should cost $400 retail when you can buy an entire desktop computer for that. Server OS, maybe. Desktop, no.
Even people like me who actually LIKE Windows 7 (and Vista) have begun to despise Microsoft immensely. You can make a good product and still be despicable company who takes advantage of consumers. Prior to Vista, I always upgraded all of my computers to each new version of an OS. Not anymore. Despite liking Windows 7, I won't buy it simply because it comes from Microsoft and I hate the idea of giving them any more of my money. So, maybe their dropping popularity numbers just reflect a lot of people like me who are fed up with Microsoft treating consumers poorly simply because OEMs are their cash cow.
In fact, I will no longer buy any retail product from Microsoft until they change their marketing tactics and pricing schemes for their OS. The repeat of the Vista "multiple crippled versions plus upgrade" tactic was just the final straw that pushed me into the anti-Microsoft camp after using MS products for as long as the company has existed.
I'm not buying their products anymore and I've been helping my friends, relatives, and neighbors setup and learn alternatives to Microsoft, as well. I also spread the word about Microsoft's ripoff tactics to anyone who will listen. The groundswell of resentment is growing. If Microsoft doesn't need consumers, then we don't need them. Apparently Microsoft has forgotten that even CEOs are consumers when they're at home, and eventually, how they are treated as a consumer will affect their corporate purchase decisions.