The moment OpenGL lost Microsoft's support, though, is when it started to die.
We may even blame Microsoft, but realistically, OpenGL wasn't going anywhere on its own. Vendors had to push it, developers had to push it, and they didn't. Same with Vulkan now, and we see the same lack of support; we simultaneously blame Microsoft and Nvidia, while the most support comes from them!
I think what we find is that gaming is its own 'game' when it comes to APIs, let alone hardware. Precision gets tossed for speed, accuracy for optimization. OpenGL was developed for the former, DirectX for the latter, in a minor oversimplification.
Note that even Apple abandoned OpenGL (and Vulkan!). I also think that we're at the point where the API simply doesn't matter as much. We've seen decent progress on live DX12 to Vulkan translation, for example. With the low-overhead APIs there's just not much room for differentiation, and with CPUs as fast as they are, the actual work that needs to be done is fairly minimal.