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.

I strongly disagree with Kotlin being strictly better than Java. It pretends to fix things that haven't been problems in Java for more than 10 years while introducing a layer of syntactic complexity that's completely unwarranted. It just thrives on hype and Android development.

Kotlin’s closed-by-default design choice makes it worse than Java, and thus not strictly better than Java. It’s premature optimization, and a design-up-front-influenced paranoia/fear of any extension in not-designed-for places. But when I write code, I prefer to keep it open to extension, and in practice, I found a lot of value in extending decently written code, that would not be possible with Kotlin without having to go back and modify things to be open.