So, "Here, use a different API with worse developer support, and in return you get access to a market that's ~1-2% the size of the Windows gaming market?"
I mean, I'm all for OpenGL. I use OpenGL. Hell, I develop OpenGL games on Linux and then port them to Windows afterwards (released ~10 made that way so far). But the choice to use it depends on what market you're going after.
Vulkan has better performance than DX11, and it has literally twice the audience of DX12 - DX12 only supports Windows 10, so any W7/W8/etc users are SOL unless you use DX11 or Vulkan.
My point was, adoption in engines is relatively slow, but once it happens, things become easier. So current usage lists aren't a reflection of general progress. Unity for example gained Vulkan support in the latest version which came out recently, and Unreal is still not there (but already close). Same goes for Lumberyard.
Of course the tax on development imposed by lock-in freaks translates into that slowness. I.e. as you said, need for engines to support many balkanized APIs means slower releases to the market.
I mean, I'm all for OpenGL. I use OpenGL. Hell, I develop OpenGL games on Linux and then port them to Windows afterwards (released ~10 made that way so far). But the choice to use it depends on what market you're going after.