Tried Kotlin, honestly I was not impressed. Tools (maven/gradle) wasn't as straight forward to setup as one would have expected. I am not impressed with the language itself and I had issue with conflicting libraries and kotlin versions.
I switched back to modern Java and did not regret it.
Java is catching up at a fast pace, I don't think Kotlin will ever replace Java or see mass adoption.
Kotlin has already pretty much replaced Java for Android devs. I see it doing something similar to Java use more generally (albeit more slowly than it did with Android).
If Google wasn't playing dirty, they would have kept updating Android Java to be in line with Java, and let the developers choose.
Instead, they purposely stagnate Android Java on a Java 8 subset, let the Android marketing machine push Kotlin over Medium articles, Twitter blog posts, YouTube Android developers channel, Android developer blog posts, MAD training material.
And lets not forget the role of JetBrains, having had a role replacing Eclipse with InteliJ in what concerns Android Studio, now they even dropped support for Eclipse on their Kotlin support, as they are quite open that they want to use Kotlin to drive InteliJ license sales.
Kotlin is just another flavour of CoffeeScript in what concerns the JVM.
I switched back to modern Java and did not regret it.
Java is catching up at a fast pace, I don't think Kotlin will ever replace Java or see mass adoption.