>Not surprisingly, c# has succeeded where Java has failed in being cross platform, used for instance in game development with the unity engine

C# succeeded in game development? That's its claim to fame? Being an optional scripting language used by Unity? The real work is done by the graphics engine that's written in C++.

>or multi platform mobile development via xamarin

Last time I checked the percentage of apps on the App Store and Google Play that were written with Xamarin tools were so low that they barely even registered.