That's exactly the point- since they're holding off on upgrading their OS, potentially for a much longer time, they won't even have the option of updating their web browser. An OS upgrade is a much more monumental decision than a browser upgrade, and the inertia is much greater.
At any rate, the fact that IE9 can't backport parts of its OS dependencies is kind of an implementation detail. We're saying Webkit has no problems providing a self-contained modern browsing experience, so IE9 doesn't have to be any different.
Webkit has no problems providing an experience that the IE team thinks is subpar (in particular, lacks full 2d graphics acceleration).
Similar for Gecko: it's doing compositing acceleration on WinXP, but not 2d acceleration.
Now maybe the argument is that IE9 should support such a mode of operation. But by the same argument, Firefox should still have a supported PPC version of Firefox 4 (even though it's JIT can't generate PPC code, so JS would be interpreted and hence very slow). Some people _are_ making that argument, by the way, but the general consensus is that this sort of decision about what sort of quality they're willing to put their name on is up to the Mozilla project. Why does the IE team not get the same courtesy?
I realize that you may disagree with their quality metrics (e.g. you may think that a modern JS JIT is a must-have requirement for a modern web browser but 2d graphics acceleration is not). But it's not clear to me that this is obvious, or that this will even be true in a year.
I agree that people need to upgrade from XP and into something better than all but I understand texel's point. Upgrading to Windows 7 or a Mac involves money, upgrading to Linux is an option that most aren't ready to take. Switching those XP users to Chrome/Firefox is the most likely chance we have to get them to a level where they can use all the new web technologies we want to throw at them. And I have a personal bias too, IE9 doesn't support WebGL which the software I'm developing depends on thus IE is not an option for my users.
So the problem is actually users refusing to upgrade their OS, not Microsoft not supporting XP after 10 years. It seems like we jump on people for not upgrading anything -- except when it comes to their OS.
At any rate, the fact that IE9 can't backport parts of its OS dependencies is kind of an implementation detail. We're saying Webkit has no problems providing a self-contained modern browsing experience, so IE9 doesn't have to be any different.