[zanny]

It is design by committee, and a huge swathe of companies are no said committee. Valve could develop an extension branch (ARB prefix means approved language features, and vendor specific would be NV, INTEL, WIN, SGI, etc. So Valve could write their own plethora of extensions as VALVE_something, yeah, and pressure the review board into accepting them into the official implementation.

[shmerl]

It sounds like rewriting the API with changing some underlying concepts is a deeper task than simply adding new extensions. (For example changing the threading model used in OpenGL).