The best way to help developers bring their OpenGL ES games to the Windows Store, would be to just rejoin Khronos and provide an implementation for the 3D API supported by the majority of the industry.
All compiler vendors provide language extensions, so I am not criticizing Microsoft doing language extensions.
However, all the C and C++ compiler vendors with exception of Microsoft tend to follow standard first and then do whatever else fulfills their goals. At Microsoft the priorities are inverted, because standard code means it is easy to move the code to other platforms.
I use Microsoft products since the MS-DOS 3.3 days, among many other vendors as well, so I am not just criticizing without experience.