XP is over a decade old. Surprised they even support it at all, being that it's basically Windows 2000 with an interface upgrade. Those clinging to it probably were the same folks saying carburetors were superior to fuel injection, too.
Think there's a few reasons why MS still supports XP, even though it is 12 years old now. Also Windows 2000 was never intended as a home or domestic use OS, unlike XP Home. Home users got Windows ME! on their new PCs.
1) Windows Vista. XP was around for a long time before Vista came out. Existing
"Designed for Windows XP" machines couldn't really run Vista properly, if at all, because of Vista's increased requirements over XP. AFAICT many people just didn't want Vista anyway, and would often downgrade Vista PCs to XP. Myself, Vista made me buy an Apple MacBook because I was so impressed with it...NOT!
2) Netbooks.
The first ones came with a Linux OS. Which obviously wasn't in MS's favour. They couldn't run Vista or 7 in a usable manner, so MS made XP available with the licensing provision the netbooks only have 1GB RAM and a single core CPU. Although in China, and other emerging markets, apparently netbooks are not so limited, they can have 2GB RAM etc, and can still be licensed for XP. Again MS would rather them be running their OS over Linux.
3) Enterprise users. Many of them are still using XP, and are only now considering upgrading to 7 or 8 with new PCs. More so in emerging markets on older PCs, which can't run newer versions of Windows. Where I am, the standard office PC is running XP with Office 2003, and is likely to be like that for a few more years yet. Looking at the ancient school property
"Founder", Pentium 4, 512MB RAM,
"Designed for Windows XP" thing I got on my desk here. That can't be upgraded to a newer version of Windows.
It runs Linux Mint OK though. 
BTW vehicles with carburettors are very popular in emerging markets and third world countries. Besically because they're easy and cheap to fix and service.
Complex computerised fuel injection systems can be very expensive if they go wrong, and spares parts might not be readily available in many regions.
Many tractors and older trucks where I am are two stroke, single cylinder diesel, i.e. very simple, reliable, electrics only needed for starting, but are not essential. You can start them with a flat battery by hand-cranking. And easy to service with only basic commonly available tools. It's KISS philosophy, Keep It Simple Stupid.
Myself, I drove older vehicles for a long time, basically because of ease of DIY servicing. And if ever on very rare occasions they would not go, it was usually very easy to find out why, either fuel wasn't getting through or there was no spark. Vehicles with ECUs and other electronics, if they don't go, you need a specialised diagonistic computer to find out why. Which usually means a very expensive garage job. I've known people write-off cars, because the ECU has failed, and a replacement was just too expensive.
You might see a 90 year old Model T still running, but it's highly doubful if any modern cars will still be running when they're 90 years old, let alone 10-20 years.
It works for me, my machine runs better with it than Vista, and that's that. Also, I don't think 10 years is a crazy long support for an OS, RHEL 6 (another commercial OS) was released in 2010 and looks like it's supported until 2023. Hell, CentOS 5 and 6 are both supported for 10 years. And it's free. 