You make a good point that much of the code base for XP was carried forward into Vista and hence Windows 7. However, there are also significant portions that have been re-written. Some of those include some security methods.

I think this whole discussion is a testament to Microsoft and their ability to keep an OS up to date and usable for so many years. We make a lot of fun of the company, but in reality, they did a great job of keeping XP able to run on current technology and with significant feature additions. In previous OS history, usually there were significant hardware changes that would have required major changes. That didn't happen with XP. Almost anything you can do with the current OS versions, you can add to XP, usually as a free download from Microsoft.

When I think about it, I'm actually quite impressed with Microsoft and what they were able to achieve.