Microsoft pushed D3D to support their own self interest (which is totally an expected/okay thing for them to do), the way they evolved it made it both Windows only and ultimately incredibly complex (a lot of underlying GPU design leaks through the API into user code (or it did, I haven't written D3D code since DX10).
The lesson though, is that APIs "succeed", no matter what the quality, based on how many engineers are invested in having them succeed. Microsoft created a system whereby not only could a GPU vendor create a new feature in their GPU, they could get Microsoft to make it part of the "standard" (See the discussion of the GeForce drivers elsewhere) and that incentivizes the manufacturers to both continue to write drivers for Microsoft's standard, and to push developers to use that standard which keeps their product in demand.
This is an old lesson (think Rail Gauge standards as a means of preferentially making one company's locomotives the "right" one to buy) and we see it repeated often. One of the places "Open Source" could make a huge impact on the world would be in "standards." It isn't quite there yet but I can see inklings of people who are coming around to that point of view.
> The lesson though, is that APIs "succeed", no matter what the quality, based on how many engineers are invested in having them succeed.
Exactly. Microsoft was willing to make things work for them. Something other vendors wouldn't do (including those who are ostensibly "open source").