We see the same argument in the cpu forum. If AMD went out of business, I think gpu prices would rise only moderately because nVidia has to sell a reasonable number of chips. As long as performance increases are as moderate as they have been lately, consumers would simply not upgrade. Now if one or the other manufacturers came out with a doubling of performance, with no competition, perhaps then they could demand an exorbitant price, especially if DX12 or 4K drive up the gpu processing games can use.
But I really dont buy into the panic mode some posters go into at the thought of AMD going out of business. Not saying I want it to happen or think it will, but I dont think it would be the disaster some people project it to be. Both cpu and dgpu markets are mature enough that prices will be held down by flat or decreasing demand, no matter what.