The car analogy is very apt and applies to the IT/support side as well. This is why, despite being an IT guy, they can have XP when they pry it from my cold dead computers. This is also why the first thing I do is make XP look like Windows 2000* (a MS high water mark for me in terms of stripped down interface simplicity and speed). Changing things up is like the Ribbon "innovation". Sure the older interface may have been a little less than optimal but after using it for years just about everyone knew where to find the things they needed. To me an OS "upgrade" should largely be under the hood and almost always allow you to use older, and usually faster/simpler, skins.
Given that a decent 5 year old PC has enough horsepower for most of the non gamers out there, the increased reliability of hardware, the economy, etc. I wouldn't be surprised to see XP maintain this lead for quite a while. It will be interesting to bet when it will slip below 50%.
Anyhow, I admin over half a dozen PCs in my house and they will be on XP for many years to come. While I progressed over the years from C64s, to SunOS4.1.1 (and many flavors of UNIX then Linux), to OS/2, to Windows; the "novelty" of a new OS has worn off a long time ago. I just want security, stability, and reliability. XP is "good enough" when configured and used correctly and when you only need it to run a browser, a few office type apps, and play movies; it will continue to be for the foreseeable future. The computer is a more mature appliance/tool today, not a toy and again the car/driving analogy works well.
*PS. This is also why the Parent could probably convince his people running W2K to upgrade to XP which is still being supported.