Well, you can use Avalonia or UNO Platform on Linux or Mac and then deploy to Windows.

UNO Platform is basically WinUI.

Mostly agreed... I'd love to see MS just ditch MAUI and/or otherwise buy out UNO or Avalonia and create a cross-platform XAML based UI library that works everywhere (including Linux). As far as Linux goes, they could just go with Gnome/GTK for underlying support... not sure if QT's license would be compatible at this point, MS mostly uses MIT for modern FLOSS.

Bonus points if it worked with C, C++ and Rust for those that want it in addition to .Net (C#).

I honestly think there's still too many old guard managers/exec at Microsoft to let that legitimately happen though. They're too busy trying to wrangle every last penny of value out of every Windows user along the way.