Well, so far that hasn't been an issue. Kotlin is a pragmatic language. The differences that currently exist can be addressed just with an annotation here or there. Also, Kotlin's features are designed whilst paying careful attention to what the Java guys are doing. Look at how records have played out. You can use JVM records from Kotlin transparently, you can create them by just adding an annotation (not that there's much point in doing so, as records are mostly a labour saving device that Kotlin already had).

Value types are perhaps a better example. Kotlin has them already with nearly identical semantics to Valhalla, but without the ability for them to have more than one field due to the need for erasure. Once Valhalla arrives, Kotlin can simply remove that restriction when targeting the JVM, perhaps add another annotation or compiler flag to say "make this a real Java value type". No language changes needed beyond that.

Kotlin is semantically so close to Java already that they aren't really growing apart, they're growing together. It works well enough to justify its usage, for me.