Actually most of the “new” (last three years) UWP Windows 10 UI is C++, and there is more JavaScript/HTML than .NET Native. The Xbox dashboard is .NET Native though and all of us on the Windows UI team are jealous.

The problem that we have is that we have so much infrastructure around C++ that it is expensive to switch. Even if you get faster compile times or theoretically less memory management bugs, you may lose the productivity in other ways if the rest of your tooling isn’t there.

With C# you definitely pay a small cost in memory consumption and a decent cost in binary size vs. C++ though, which still matters.