My understanding is that fixing newly discovered vulnerabilities in Windows XP or Windows Server 2003 would be fairly inexpensive.
One more downside to being closed source - if Microsoft won't fix vulnerabilities, no one else can for any sane price.
At work I'm still migrating our last two 2003 servers, one migration nearing completion the end of this month, and the next not even started yet but expecting to take 9-12 months.
Exchange server was our primary risk because by its nature it has to handle SMTP, and while you can't poke that server directly from the Internet (a postfix relay server is the only one with direct internet exposed ports) but those emails still flow through it, and it sends outgoing mail directly so has to connect to other MTAs and everything involved with that like DNS queries... A pretty big risk footprint on that one, so no argument from me that it needs upgraded.
The last 2003 server however doesn't technically require being replaced, the risk is very small and mostly controlled for even then. It would likely run fine until enough hardware failures make keeping the server up cost prohibitive, which is really the biggest reason (though a fairly justified one) to upgrade.
The vulnerability risk footprint is limited to the LAN, and then only really to windows file sharing (that and SQL server are the only exposed services)
Not zero for sure, but taken alone not enough of a reason to justify the cost of an upgrade. Only everything taken together combined with a string of purchase approvals to upgrade everything else that demands it, is why it ultimately will be.
If only another big player could release continued security updates, or ideally more than one to help both competition on price and a choice of whom to trust for such a thing.
There is definitely a market for very long term support, which you have to look no further than IBM to see.
In fact many would trust IBM to fill such a role if they were to do so. Others may trust Google. I'm sure there are plenty of other examples as well.
But I don't see "long term windows support" being in many of those companies interests, nor see microsoft going along with such a plan even if they were.
Microsoft wants you to buy their latest shiney instead, Google would prefer you didn't use Windows at all, and IBM doesn't seem to be as big on the support thing these days even for their own products let alone microsofts.
All of those facts factor in to the cost of providing security updates, and does raise the bar quite a bit higher than it would appear at first glance.