And game availability relates to GPU API support available at the platform... how exactly?

The fact that game studios decide to use one API or another is no factor in evaluating API availability. (Also note how many DX12 games not published by Microsoft are DX12 exclusives - that says something about Microsoft motives, but not about API qualities).

There's a reason why DX had almost zero adaptation outside games. Maybe it is not that good API, if you need to support your app for a few years too. Games are fire and forget.

Actually 2D and text rendering, WPF rendering, UWP Composition API, UI animation support.

All of them used by most desktop applications on Windows, including image, video and 3D editing applications.

Not explicitly. It is used by platform libraries that these application use. Given that they are provided by the platform vendor - the same that pushes DX - it's no wonder, that it was being pushed there.

CAD, GIS, or other applications that need to use 3D explicitly, on the other hand.... Basically only Autodesk ventured there, with 3ds Max and AutoCAD (since 2015).

Well, Autodesk products are market leaders, probably Catia is the only alternative.

There are actually many CAD packages, with different levels of complexity, from Siemens NX to VariCAD, plus myriads of domain-specific ones, like ArchiCAD.

For example, most of our customers prefer MicroStation, for some reason (no, we are not Bentley shop ;).

I'm sure you can see that 2/5 (approaching half) of the Direct3D 12 titles were published by Microsoft themselves.

And actually, by the numbers there are more titles on Vulkan than there are titles not released by Microsoft on D3D12.