All video cards support DirectX - all. Not all natively support OpenGL. All the libraries you mention support the reason for using DirectX - why use twenty libraries to accomplish what one API can do? And just because my game does not support OpenGL or does support OpenGL does not mean it will be successful or a flop. That is not even a fair statement. Games do well based on whats in the game, not what library it uses to accomplish what it does. I'm telling ya, the days of proprietary graphic API's for certain video cards are coming to an end. DirectX will be all there is. 3Dfx API fell by the way side even though it was easier to use and had less dev time than DirectX. Granted 3Dfx fell for a host of other reasons not related to this topic but you get the jist of what I'm saying.
Well, If you think that 8% is not worth programming for, you're software will not that successfull as if it used OpenGL. It seems to me that everyone just thinks DirectX is the only possible solution, and it handels input, sound etc. Well, this can also be accomplished with various of libraries like Allegro, ClanLib, SDL.