Why would anyone expect that WinXP's global market share would suddenly go into a free-fall just because Win7 is now available?
The average consumer doesn't upgrade Windows at all. Instead, they buy a new PC whenever the old one finally dies or whenever it is no longer able to perform the functions they need it for. If a new version of Windows is being distributed at the time, then it ends up on their new machine. That's how new versions of Windows get 'adopted' by PC-buying consumers.
Furthermore, Windows adoption and market share are no longer mostly determined by the buying trends of Western consumers. Over the last 5 years, consumer PC ownership went from 1 billion to 1.7 billion PCs and it was mainly due to rapidly increased adoption of PCs in developing countries. Prior to 2006, nearly 60% of all PC users lived in the West, but since 2007, 70% of first-time PC buyers lived in developing markets.
The developing markets, which now controls a large chunk of the XP install base, are going to be much slower to adopt Win7 because there simply isn't a pressing need for a new PC or the new Windows interface. Hardware still comes with XP drivers, and new software versions are still being written to be XP-compatible. Most consumers, particularly those in developing countries, bought their machines to gain access the web and run basic software. Any 'old' machine will have more than enough processing power to handle the demands of what constitutes general usage today.
The bottom line for the average consumer is this: If you can still fire up your machine and run the software that you would have been using on the new version of Windows, there's no compelling reason to switch versions of Windows. The software is ultimately what you're actually spending time using, not Windows.