I'm not sure where you got that impression, but WinUI3 is in almost all of Microsoft's software at this point. Admittedly a huge amount of it is embedded inside React Native, but WinUI3 seems to be the UI toolkit that has won and is still going strong. They announced a bunch of WinUI3 enhancements at BUILD even.
(One such announcement is a coming standard control for the sort of custom Windows Title Bar with a search box and branded logo and extra buttons that has become common in every first-party application from the Microsoft Store app to all of Office and many other places.)
> Contrary to this article, it still doesn't even have a visual designer.
Huh? Visual Studio has a visual designer for the WinUI 3 flavor of XAML. If you don't want to write XAML to back your UI they probably will never support a Visual Designer for you (ie, C# UI seems neat and maybe a bit more "functional programming", but I'd rather use XAML just to have the Visual Designer at least). Last I checked the XAML designer works just fine even in C++ projects targeting WinUI 3.