There are so many other good alt browsers out there....it is either MS come up with an XP version or Chrome and FF fill the gap. That's all there is to it realy.
There is no value in investing on Windows XP. Sooner or later Chrome and Firefox will just stop supporting that platform as they too will leverage OS functionaly that enhances performance and user experience that isn't available in XP. IF you want to hold out forever then those browsers are you chance to do so.
Just as chrome and Firefox generally don't support OSX < 10.5 moving forward and are intel only.. Firefox seems to quietly be dropping a lot of platforms
If that is not a good enough reason for Microsoft to come up with an XP version (and fast) then maybe they deserve to lose the market share they are currently losing and are about to lose even more.
XP isn't generating marketshare and any loss of marketshare is a new opportunity for windows 7 which is a profitable enterprise for Microsoft to develop for.