Re: Are old XP machines being upgraded/replaced, or simply retired?
I'm in the same boat - have an old frankenstein desktop machine at home which is basically built out of the remains of a few others (the case says "Tiny PCs" on it, in case you want a heritage yardstick) and it chugs along nicely under XP. It does what I need it to do, getting booted up roughly once a week for a few odd jobs.
For the day to day stuff there are a couple of laptops around the house (running Win7) plus a few tablets (Android) which more than suffice. So the old desktop machine just provides a bit of a back-up in case of need, but I can neither really justify updating it (I doubt the hardware would support Win8.1) nor justify chucking it (as it still works fine, if a bit slow compared to its newer siblings).
Hence it'll probably continue to rumble on in the background until it does have a catastrophic failure somewhere, but until then why not? It is fully patched up to the cut-off, and has AV/firewall from another vendor who are supporting it still (and it's also behind a router firewall anyway, and it doesn't venture off the beaten track to more dodgy areas of the web).