DX10 can be ported to Windows XP, technically

The original reason was that DX10 required graphics memory to be virtualisable. This would not work with XP, and that was fine and dandy. It was an honest technical reason why you could not backport DX10 to XP without a major rip and replace operation. Then something odd happened. Nvidia had about as much success implementing this required feature as it did with it Vista drivers, that is to say, none. So, MS threw NV a life preserver and made GPU memory virtualisation completely optional. In doing this, MS removed the only impediment to backporting DX10 to XP, it is now, and has been for quite a while, completely possible. MS is screwing its customers to force an upgrade and you are a pawn in their revenue generation scheme.