We are talking about being compatible with two API are that are completely different. D3D11 don’t give much compared to D3D9 and is similar in the code and required ressources to maintain. Plus most of the gaming park that was compatible with DX11 IS compatible with DX12 (Windows 10 have less overhead than Windows 7 and is overall better. Plus it got free update… The quantity of user that are stuck with DX11 should be lower than these with DX9 because they got a faster and better upgrade for free).
Talking of statistics, the desktop Linux market share is going up really fast (for some unknown reason, maybe some people really don’t want Windows 10?)
May 2017 - 1.99%
June 2017 - 2.36%
July 2017 - 2.53%
August 2017 - 3.37%
It’s technically a superior OS and they are not living in the past because they only support D3D9 natively, it’s more of a patent concern and available hackery as official card maker don’t try to release these API on Linux to not get sued. But Linux, as Windows 7+, support Vulkan… That is the present. D3D is the past, Microsoft released DX12 to counter Vulkan, but it’s not better. Vulkan is available for 99.95777% of the gamers (I invented the 0.04333% for apple)
I don’t talk about Apple here because they have a superiority complex that prevent them to have any decent 3D API.