Not saying it's valid, just that SE will hide behind the fact that Windows XP exists at least until year 2024.

May 2011 valve hardware survey:

Windows 7 64 bit 40.64% +0.54%
Windows XP 32 bit 21.63% -0.90%
Windows Vista 32 bit 13.73% +0.39%
Windows Vista 64 bit 12.13% +0.61%
Windows 7 10.44% -0.68%
Windows XP 64 bit 1.00% 0.00%
Windows 2003 64 bit 0.33% +0.05%
Other 0.11% -0.01%

http://store.steampowered.com/hwsurvey/?platform=pc

As you can see, only 20% of surveyed PCs are non DX10 capable either because of hardware or software.