I can't believe that after how many years, people still repeat this fallacy..
The reason why APIs sometimes require an OS upgrade, isn't because Microsoft is just trying to squeeze more money out of consumers..
It's because the new API comes with a driver model update as well. Vista had a completely new driver model that added a number of new features and enhancements that increased the stability, performance and capability of the display drivers over that of the XP version..
Could they have back ported it? Possibly, but it wouldn't have been worth the effort..
Same thing with Windows 7 and Windows 8/8.1. Windows 8 came with a new driver model