This is a reasonable critique of other JVM-hosted languages I've worked with (Clojure and JRuby), but the external interface of classes and methods in Kotlin ar

This is a reasonable critique of other JVM-hosted languages I've worked with (Clojure and JRuby), but the external interface of classes and methods in Kotlin are completely compatible with Java by design. And in practice, it's very easy to write mixed Java/Kotlin projects where you don't necessarily know (or care) whether the class you just imported was written in one or the other -- it just works how you'd expect. I'd be interested in specific examples where you found any impedance mismatch at all.