Totally clear that mainstream desktop is Windows, native and .NET but the question again and since there are rarely new apps on Windows: who is actively developing new desktop apps in C#, .NET on Windows which are not games?
Almost everybody doing in-house enterprise desktop apps (and those are a heck of a lot) and tons of people doing desktop Windows apps that don't need to be C++ (e.g. not Photoshop, Office and co).
Again, those are in the tens or hundreds of thousands. Check any Windows app review site or repo for examples.
It is _massively_ faster to write a quick UI in .Net than it is to write one in HTML.
I have been doing WPF development in the last two years for enterprise customers.
Also as a Windows user, there are plenty of applications to choose from.