Kotlin is by far my favorite language-to-date. I moved my dev team to it years ago, it's all server side. We still have over a decade's worth of Java code to maintain, but anything new is in Kotlin (even in the legacy applications, we just add Kotlin). So nice. Makes me happy when other people get onboard and say "oooh yes, this is much better than our legacy Java". I was a HUGE Java proponent from 1999 to around 2010, when I started really getting annoyed that the language was missing so much that other languages had. Eventually we tried Scala, too complex to deal with in large teams to get everyone onboard, Clojure was a pain as everyone tried to be as clever as possible (and errors were awful, tooling was painful as not everyone wants to use emacs), then Kotlin came around and we never looked back.

Kotlin on the server is growing though and has the biggest library ecosystem to exist

Glad to hear this. There's really no reason it for it not to do well on the server.

Pattern matching is (will be) better implemented in Java. It's fully fledged compared to Kotlin's implementation.

When will the last-argument-as-block be implemented in Java?

This one brilliant feature allows for easy DSLs without macros, parsing, or otherwise introducing new concepts to the language.