Hi Torsten,
This has been a point of contention for awhile now within the OpenXR community especially because those people who have Microsoft WMR headsets will have the WMR runtime as their default and WMR only supports Direct X and many game engines focus on OpenGL and Vulkan (as Godot does) and are therefor not supported by WMR (unless you go over SteamVR).
The problem though is that the OpenXR spec dictates that it is the user who decides the runtime. The idea being that you should write cross platform implementations.
So the "official" stance is that you're just going to have to instruct your users to make SteamVR or the Oculus runtime their default.
Indeed, the editor has a dropdown which simply manipulates an environment variable that lets you temporarily override the default, it's a concession they made as in development you often want to test your game against different runtimes. The problem is that you need to know where the runtime is installed as the environment variable is set to the path where the runtime is installed at. If users have selected an alternative install folder it won't work and there is no way to retrieve the available paths.
There is now a mechanism to discover the installed runtimes so we could potentially come up with a mechanism that validates if the active runtime is supported and switch to another if its not before initializing OpenXR. But currently that hasn't been implemented.
