I'd argue that just because a language runs on a platform with a potentially large number of third-party support (JVM) doesn't mean it's going to be nice to use.
For example, I ended up having to write Java code when dealing with large XML data sets in a Clojure system, because the current offerings simply didn't let me do what I needed in fluent Clojure code. I suspect Kotlin has those same kinds of issues, just amplified because it's a lot less widespread than Clojure.
The fact that it integrates with Java isn't what I'm getting at. Developing in Clojure eventually forced me to write some Java, which I didn't want to do. I just don't see Kotlin as somehow solving that problem.
For example, I ended up having to write Java code when dealing with large XML data sets in a Clojure system, because the current offerings simply didn't let me do what I needed in fluent Clojure code. I suspect Kotlin has those same kinds of issues, just amplified because it's a lot less widespread than Clojure.