No one would go to Linux just because OpenGL would become the gaming standard, because if it did it would only replace Direct3D. DirectX is a all-in-one integration package that supplies more than video. Controllers, audio, video (2d&3d), etc. Developers love DirectX because they can spend more time making content and less time worrying about how to make everything work together.
Also, MS provides DX support free of charge to developers...no licensing fees and a wide footprint means no brainer at choosing a platform to support. Another strike against linux is its strength. Developers have enough problems developing for one platform with all of its various hardware options...imagine having to deal with all the various distributions out there, with varied package systems, varied kernal compilations.
MS provides standardization as well as a large install base....linux might be growing, but until it starts showing up on average Joe's machine it will never be more than fleeting potential.