What's your point? That's what Apple makes available. I'd use the C# API if that's how they provided it.
If not dominating the games on those plarforms, Unity and C# have a strong footing to say the least. Swift doesn't seem to be making very much headway on platforms where APIs are available in anything else.
Maybe that can chance. It seems like a neat language but "it's popular because apple forces you to use it" is more damning than reassuring.
The point is that they are guest languages on Apple ecosystem and need Apple tooling and languages as means being available.
I may also add that I dislike Microsoft doesn't give to the .NET ecosystem the same care for games developers as Apple does for Swift and existing OS SDKs.
As far as DirectX team is concerned, only C++ exists, and .NET team lets third party folks do the needful.
Had it not been for MonoGame, Unity would never picked C# in first place, gone were the days of Managed DirectX and XNA, when the decision came to be as Unity did their cross-platform rewrite out of OS X.
The specifics of C# are fairly irrelevant. Point is that even if swift is forced, middleware can and will just plaster over that. Even if Metal is forced, tools can plaster over that.
Apple forcing an API is not enough to sustain a language's popularity.
When the language is required for one of two mobile ecosystems, and second major desktop ecosystem, popularity is relative.
For decades C# was only relevant on Windows, outside Unity never got wide adoption among AAA studios after Unreal became free, and after their license debacle less so, Godot favors C++ and GDScript even with C# support it isn't what most folks reach for, and Microsoft keeps having an adoption (popularity) problem on UNIX culture oriented startups.
While just like Swift on Apple's ecosystem, C# is doing just fine on Microsoft culture environments.
