There are always going to be people resistant to change, and of course there are always people out there who like messing with old stuff.
I love old cars, some people love old computers and operating systems.
While I believe the world would be a better place if we all drove Triumph TR6's and Lotus Elans, it's only a dream. The reality is I've got all of my friends driving Honda's and Toyota's because they are safer, cheaper and more efficient.
That's not a valid comparison IMHO. Old cars are slower, pollute more, less safe. With XP I can do everything I want to just as well as with Windows 7, without extra DRM, warnings and authorisations popping up, it's perhaps faster than Windows 7 or at least around the same speed, and while XP may be in theory less safe i've never had a problem caused by a security problem with Windows. Because XP's smaller and older it's faster on some hardware.
Let's turn it around, given XP is smaller, faster, and works perfectly why would any sane person pay money to have a slower computer than will let you do exactly the same things? It's an OS, it's not like upgrading to the next version of photoshop, it's plumbing.