On the contrary, as someone that earns their bread using both ecosystems, .NET is still pretty much tied to Windows, regardless of .NET team efforts, most shops going with macOS or Linux are former Microsoft shops saving on server licenses or giving Macs to their devs.

There are many platforms where .NET doesn't have an implementation, a phone to call their own (even if ART isn't proper Java, WP is no longer around), embedded systems, including factory and military weapons deployments (PTC, Aicas, microEJ), copiers (Xerox, Ricoh), phones (Cisco),....

C# is definitly better than Java dealing with value types and low level programing, or being embraced by the game development community, however not sure if the featurities of last years is the right path, I am starting to feel I should just reach directly to C++ instead.