The whole "XP is obsolete" thing is a red herring. Actually there are 2 red herrings implicit in this idea.
Red herring No. 1: That this is intentional on Apple's part. There is absolutely no evidence to support that, quite the contrary. Support would not spend 3 hours on this with someone if the answer were as simple as, "Sorry, we stopped supporting XP on April's Fool Day. Goodbye." It would not be unannounced. So all the arguments as to why Apple MAY decide to drop support for XP at some point in the future are valid, but just don't apply in this case. It's a BUG, probably caused by sloppy testing on XP since they probably put their least qualified testers or fewer resources on XP testing. Or that it didn't occur to anyone that since the client-side software (iTunes) didn't change, only the protocol did, that it could possibly be OS-dependent (and in a well-designed system it shouldn't, but nothing is perfect). Now whether Apple chooses to "resolve" this bug by retroactively claiming obsolescence is another matter.
Red herring No. 2: That it is an issue of continuing support for XP in the face of Microsoft disowning it, etc., etc.
(a) iTunes didn't change in the time before and after the problem started manifesting itself, and iTunes itself is advertised compatible with XP. (At some point in the future it surely won't be, at which point you stay with the last version that does, until you decide you can't live without something new that's only in the newer version, just as with everything else you buy, but that's not the issue here.)
(b) Apple owns both ends of the interface between iTunes (on whatever platform it may be) and the iTunes Store server, with something called the Internet in between. Something about that interface was changed on the night of Apr 20, and was surely intended to be transparent (otherwise some announcement to that effect would have been made, or uncovered by now). That change turned out to have a (surely unintentional) OS dependency on some supposedly standard protocol handling that's done by the OS, but turned out to be done differently in XP. I don't know enough to speculate as to what that dependency may be, but others may. At any rate, once it's identified, solutions or workarounds, whether proposed by Apple or some clever hacker, should emerge (or so one hopes).