As a Windows 10 user with NVidia graphics, I was never too happy about the fact that OpenGL was the default API, as DirectX 12 would have certainly seen improved performance. It did however make sense to have OpenGL as a base for PC to support multiple platforms as well as older Windows OS versions. Also keep in mind that NMS was first released in 2016, when Vulkan had its initial release only a couple of months prior.
We are now a few years later and Vulkan has rightfully grown in popularity, with broad support and being an open source low level API. I am sure HG has been following closely and eventually made a decision to make a change. When NEXT was planned, including a release for XBox, HG was more or less forced to port/compile to DirectX 11.X/12 as well. So by this time they had OpenGL (likely their coding base API) for PC, GNM(X) for PS4, and with NEXT, DirectX 11.X/12 for XBox.
I am sure all the AMD complaints together with plans to go VR and further development plans requiring performance improvement, has led HG to rightfully drop OpenGL completely. Going Vulkan makes for that matter most sense, as it supports pretty much the same hardware as OpenGL, but either on par (GNM(X)/DirectX 12), and in many cases with improved performance (AMD/pre-DirectX 12).
Now HG can continue to port for PS4 (GNM(X)), and XBox (DirectX 11.X/12), while likely changing their base API to code for being Vulkan. They would no longer have to port from OpenGL to other required APIs, where porting from Vulkan to others is relatively easier. This to me seems like the best choice going forward and likely ‘required’ for whatever HG has planned for us.
Looking forward to upcoming PC hardware, as well as new consoles, the only odd one out, appears to be Microsoft (as usual). I believe Sony is likely planning to support Vulkan, where I have great doubts that the XBox Two will provide this. Considering both will use AMD, it would make perfect sense to have Vulkan support, although keep in mind this is ‘custom’ hardware, where Vulkan will not really improve performance, if at all. So worst case, HG will have to continue porting to both platforms. I do however hope consoles will follow, considering their use of AMD, just to make it easier on all of us.
Then of course we see other development, like Google’s Stadia, running Linux with full Vulkan support. Not to mention some other platforms like Nintendo Switch, Android/Mobile who offer full Vulkan support as well. Clearly OpenGL has to go, it is outdated and has no future, unlike Vulkan.