Dead is perhaps overstating things, but I think Xamarin is problematic for new development because it hasn't kept up with the latest movement in the iOS ecosystem.

Looking at their issue tracker, it seems watchOS development is a nightmare, there's no support for catalyst, and there's no support for WidgetKit. Widgets are very hot right now, so that was disappointing.

It used to be that if you could make an app for iOS with Swift, you could make it with Xamarin. That guarantee is no longer there.

Google will move on and then all of the developers who hinged their platforms on it will need somewhere to go to.

You seem to be correct (in so far as Firefox is also dead): https://trends.google.com/trends/explore?q=Xamarin,%2Fg%2F11...

I don't know about that. Pretty much all cross-platform solutions are a rounding error compared to the first-party Swift/Java platforms. I can believe there's potential for another yet.

Xamarin is huge in the .NET world and Microsoft is investing heavily into it. I know plenty of LOB apps, huge apps, built in Xamarin. I don't even know of a single app built in Flutter.

It's ok but not huge. I'm starting a second mobile app in native Xamarin and there are some pretty active communities around.