Soon after the graphics card industry would go into a slump, starting a final death spiral for PC gaming.
Why? Suddenly we're all going to get braille screens and not need a graphics card any more? Yeah sure, we won't have a hundred different variations on a theme, but 3d accelerated graphics are here to stay. An awful lot of serious applications need them, heck in the case of Vista, the Operating System needs them. If anything, it would be easier to develop if there were only half a dozen different cards out there. You'd have a target which wasn't moving quite so fast. And it's not like (many) indies are targetting 8800GTX as minimum spec as it is.