Not sure if OpenGL 4.0-4.6 is less work in general. I'd argue implementing Vulkan gives the best ROI, since it gives all OpenGL after that without extra effort.
And I don't think that few applications use it. Anything modern tries to. The rest are planning to move to it.
And I don't think that few applications use it. Anything modern tries to. The rest are planning to move to it.