How much performance would a pure and thus leaner driver gain, I wonder.
Also a fair bit of backwards compatibility will be involved, like with x86 cpus. And even then, the same fixup is required to run opengl on directX cards, I believe.
Considering that even with API's that have some what good internal compliance testing like DirectX developers still ship utterly broken and un-optimized code - none.
This is also why Vulcan will probably not succeed (at least now how people think it will).
The last thing that say Nvidia wants is to maintain a code base of 3-4M LOC's which 50-60% of it is intended to allow for games to run anywhere from running at all to running well.
With how the current market works the driver is the "secret" sauce that GPU makers use to compete in the market and is just as important (or even more in some cases) as the hardware it self.
