My understanding is the WinUI 3 Win32 model is still going to be basically doing what you can already do with XAML Islands, just packaged more nicely with wrapper classes and Visual Studio templates and so on, so by itself it doesn't change the underlying HWND vs. CoreWindow situation (just wraps it). There are already Win32 apps, like Windows Terminal, that implement a "UWP" UI by doing their entire UI in one big XAML Island.