Only a small number of programs and utilities that ship as part of Windows uses WPF. Windows has more WinForms-based GUIs in it than WPF-based GUIs.

It’s a far cry from the Longhorn demos of 2003 that showed the entire Windows UI been made with WPF, and Explorer itself hosting third-party applications’ WPF components.

Yeah, I meant WinUI/UWP XAML. Starting with 8.1 it's been the standard for any new or overhauled Windows shell UI, along with the built-in apps. As you mentioned, this was the original intention for WPF, and the failure of this plan (which the Windows team largely attributed to the dependency on .NET) was the main reason WinUI/UWP and WinRT came about in the first place (as basically a do-over of the Longhorn plans, but ditching the .NET dependency to build on COM instead). So it succeeded at getting Windows to adopt it (at least for a while - apparently in 10X they are starting to use web tech for shell pieces like the new Start menu, although I think the actual window manager itself is still XAML based), while WPF failed.