I worked for a startup that used Kotlin. The language itself is not that bad (if you ignore the engineers that want to use every single time all the features the language has to offer), but the ecosystem was awful. We were using Gradle, and Spring Boot: just too much magic for my taste. Gradle itself is a beast I never fully understood, and I don't comfortable working with such tools (the Gradle userguide has over 10…
JVM's advantage has not been "Write once, run anywhere" for a long time - we all run code in docker containers anyhow. The real advantage is stability and tooling availability