It depends where you are. My company switched to Java from C# because the right executives heard about open source. If you ask most managers around here C# might as well be COBOL and Java is a brand new language( certainly not something from the 90's originally intended for cable box menus).
So there are certainly bubbles where C# is dying if only for stupid reasons.
AFAIK .NET Core offers nothing of any value for cross-platform GUI though. There is Xamarin.Forms but last I looked at things it was pretty shit compared to Java's offerings.
Xamarin.Forms isn't really equivalent to any (commonly used) Java options. Something like GTK# or WinForms+Monk would. There's also Eto and Avalon "in the pipeline" so to speak.
To me neither Java nor C# does well at WORA UIs but they are at parity imo. JavaFx isn't bad, but it doesn't do anything the C# offerings can't do fairly easily.
I'm actually a fan of Qt for true cross-platform apps these days myself