So, it gets stressed that this doesn't have a dependency on xamarin.Forms. Ok, but what's the advantage of this versus using the UI building capabilities included in .NET 5 directly?

To paraphrase - what's the selling point of using Uno vs using .NET 5 and .NET MAUI (the successor of Xamarin.Forms) directly?

From just glancing at it, it looks like this is targeted at developers creating WinUI applications and turning those into cross platform applications.

WinUI being the UI platform that Microsoft seems to be pushing on windows these days.

Is there a guarantee that this is not going to share the fate of WPF?

Like still being maintained and supported on latest Windows and .NET versions?

The problem is they never truly iterated on the product. There are a hundred ways you could simplify and improve WPF.