If you're using legacy hardware, presumably you likely own the legacy OS (XP) already that runs it, and hence don't need to worry about being able to buy XP in the future.
I can't think of any reason why one would care if XP sales are discontinued if one already owns a copy and
it is working for one's legacy H/W. I guess if you're unfortunate enough to have an OEM PC that ran XP
but came with NO real OS installation CD and then that system breaks one can end up screwed over and
needing to buy a copy of XP so one can use it on a replacement motherboard along with one's legacy
sound card / scanner / webcam / printer / joystick / network card / whatever. If one is likely to be in the position
of needing an XP install CD that one doesn't already own then, yeah, it'd be good to get one from ebay or a vendor
or something ASAP.
If you're worried about the discontinuation of *support* and *security updates* for XP, yes, that's certainly a valid
concern for a production system especially if it is exposed to the internet at all. I suspect the major anti-virus and
anti-spyware companies will continue to actively support XP with their newer updates better and longer than
Microsoft may. Fortunately at least new web browsers like Firefox will continue to be updated for XP to help
improve the security risks there. Ditto for things like email clients, office / document software, et. al.
Personally I just bought a few copies of XP in a close-out sale for way below retail cost and I figured they'd
be enough for my needs until I just didn't care about XP anymore. These days it's more likely that I'd be interested
in running it in a VM to handle legacy software than I'd be interested in running it on a newly setup PC.
Also LINUX is doing a good job supporting a lot of the lower powered systems and legacy devices that Vista wouldn't run well with. So if it meets your needs, it can be a good way to use existing legacy hardware and lower capabilty
old PCs and put them to good use. With WINE you can even run a useful subset of XP software under LINUX.
Vista is "ok" IMHO and runs tolerably well on newer relatively more powerful hardware, but even so, I am
minimizing my use of both XP and Vista and migrating more and more of my daily use over to LINUX whether
on newer or older generations of systems. When that fails, there's always a VM of Vista/XP/NT/whatever.