I don't think that's necessarily true. Dotnet is still just getting started on Linux, meanwhile the Java ecosystem is insanely mature on Linux. Fwiw, I have been writing dotnetcore lately, and enjoying it, but it definitely doesn't match the java ecosystem in terms of library maturity. Because of Google/Netflix/etc, Java on Linux is a really interesting beast

Not to mention that toying with another language is not such a complicated endeavor once you're 5-10 languages in. The great thing about C# and Kotlin is they've been good at adopting features from other languages that are already good and familiar, so picking them up is generally super easy.