[Macha]

I don't think it can be Kotlin, but I don't think it needs to put a serious dent in Kotlin adoption, as it just needs to stop driving people away so hard and then the friction to do something different will keep people in Java if Kotlin is say, only 30% nicer. See for example coffeescript getting limited by ES6.

[vips7L]

IMO Kotlin is barely nicer than Java currently. Java has for the most part implemented better or equivalent versions of a lot of Kotlin's features (e.g. virtual threads, pattern matching, sealed classes).

The last things missing are properties (which should be coming with reconstructors), and nullable types. For me, nullable types are not worth the switch.

All that being said, I don't think the languages are mutually exclusive. As a JVM developer you retain all your knowledge of the JVM and it's standard library. Lastly, a language doesn't have to be the dominate language on a platform to be usable and employable (just look at Scala).