Disclaimer: I haven't used Kotlin. I know very little about it. Take all this with a pinch of salt.
COBOL's still around, so I doubt that anything will completely replace Java any time soon.
One aspect of the "Kotlin replacing Java" side of things is that I'd expect most Kotlin developers to still need to know Java, at least for a long time. You're likely to be using libraries written in Java, with Java examples and Java-based questions on Stack Overflow. Maybe libraries will come with Kotlin examples as well over time, but I suspect that relatively few will come with only Kotlin-based resources. (Is this true for Objective-C/Swift? I suspect not, but again, that's a comment from a position of ignorance.)
I suspect that the health of the Kotlin ecosystem will at least somewhat depend on the health of the Java ecosystem for a long time too. It'll be interesting to see how big "Kotlin not in a JVM-like environment" becomes.