Why do I say this? Because the migration from Xamarin.Forms to MAUI was (actually, _is_, as it's not really 100% finished yet) a horrible PITA.
Let alone the migration from .NETFramework to .NETCore (now called "dotnet" since v5) which delayed us months if not years (but people that used Avalonia would be affected by this too; so the way to dodge all this would have been choosing Flutter or ReactNative, I guess).
That being said, the main reason why I chose Xamarin.Forms over Avalonia at the time is because the latter didn't have Mobile support. How's Avalonia's mobile support now? Even if the port is 100% complete, I always thought "well, XF/Maui are centered around mobile, making desktop just be an after-thought", which to me is an advantage because I prefer better UX for mobile by default, and require devs to spend extra effort for desktop support (instead of the other way around); given that desktop usage of apps is declining day by day.