Java is still the only JVM language with more than 1% usage on any industry ranking of languages.
All non-JVM languages combined represent 10% of JVM usage
https://snyk.io/blog/jvm-ecosystem-report-2018/
As someone who writes a lot of Kotlin for a living, something like 80% of the improvements Kotlin brings that I use on a day to day basis are features to give it the same level of ergonomics as C#... like reified generics...
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And your comment that .NET is a de-factor one-language platform makes it sound like you've never heard of the DLR (or F# and VB for that matter)
To me the biggest reason DLR languages are not as big as things like JRuby is C# is a pretty damn good language. There's much less value is trying to cobble together existing languages and subpar runtimes when the defacto language is modern, developing at a steady clip, and "delightful" to use.