Kotlin is a few tweaks on top of Java, most of which aren't relevant anymore, and it's not strictly better in most ways other than saving a few keystrokes (and preference).

It's a little bit nicer to write but that's almost irrelevant.

It also comes with some runtime cruft.

In reality there is no Kotlin without Java, which means most projects end up a bit 'dual'; every single Kotlin project we've had (except Android) folded back onto Java. Even Scala wasn't worth it, though that's a different question.