The simple counter argument to all your ranting is dx11 which is only available on Windows 7 or later and contains api:s not available in dx9 and dx10 which means blizzard maintains multiple versions of the engine one for dx9 and one for dx11 and with the change the dx9 can slowly disappear and in the new expansion dx11 is the new lowend engine since every computer above 7 supports dx11.
The reason they say xp and vista vs dx9 and dx10 is because non techies know of xp and vista but not dx9 and dx10.
So i'm sure this is heavily a technical decision communicated in a form common to both technical and non technical folks.