I don't see XP as a big enough problem to upgrade anyway.
Firstly, yes, you lose updates, but on that size network, you're going to be running via a central router/firewall/filter whatever, enough that *direct* attacks aren't possible or at least minimised.
Indirect attacks (downloaded malware) is a serious risk in almost any environment, but there's no reason you can't lock down XP pretty much like later OSs, not to mention virus/malware scanners and such.
You lose support. Or at least free support. Big whoop. I have a real problem with the "support" supporters, (excuse the pun), namely because I've never seen anyone call support and received a *useful* answer. Not talking about stupid non-problems you can google, but real problems. Maybe I've led a sheltered life, but anyone I know who's called for real problems has hit a brick wall with Microsoft. In other words, you've lost nothing, because you didn't have it in the first place.
Software requirements. Believe it or not, sometimes *YOU* have no control over what software someone else has picked. You know the one, it ONLY works with IE v6, you're in charge of keeping it working, the Powers That Be won't let you replace it and you can't argue with the idiot who bought it - usually because he's your boss.