"Try using a 13 year old install of XP on a 13 year old PC and you'll find that it doesn't work...very well at all."
It works fine if you have the original configuration - and typically that means no AV and anti-spyware running, and not piles of software that no one really uses but each one starts its own updater on boot-up. If not running well, usually some RAM as an upgrade is enough to restore sanity cheaply.
However, I think you will find there are two classes of hold-outs where the machine is newer but they stick with XP:
(1) Folk without the income or desire to spend on a PC, the old "it works fine for me and I hardly use it anyway" brigade. Definitely not El Reg readers.
(2) Folk who have the budget and support, but are sticking with XP because something they have won't run acceptably on a newer OS. Now you could argue they should simply upgrade the program(s) they use to avoid this, but there could be a whole range of reasons why not:
2.1) Stupidly expensive to do as it was custom software, etc. (thinking here of gov and IE6 lock-in, for example).
2.2) Not possible as no newer software exists (e.g. for old hardware, or company went bust, etc)
2.3) The upgrades change things in ways that will (or could possibly) break something key to their business (e.g. industrial control where it took a lot of time & money to certify the system in the first place).
All can be sorted with enough money, but it is likely to be WAY more than the cost of a new PC/OS.