I used to like Kotlin, but now I switched back to Java. The only thing that I miss in Java is short property syntax. And praised null-safety just gets in the way too much. Kotlin adds new layer of complexity to tooling, etc. I don't like some Kotlin idioms. Kotlin made a bet to features I don't need (Kotlin native; coroutines). Kotlin compiler is slow comparing to Java.

That said, Kotlin is awesome language with awesome tooling. I don't think that it'll go anywhere since Google declared it as a preferred Android language. So you won't make a wrong choice, it's just a matter of personal preferences. I'm more of old guy who's not afraid of writing XML and repetitive code and don't like all that modern functional one-liners.