I think it's never be 100% Java interoperable.

If you want to eliminate billions dollars mistake (NPE) in Kotlin but still need to interact will Java code, then you will need to use @Nullable / @NonNull annotation in your Java code. Unless it will cause Kotlin NPE.

And when using Kotlin with Java, it will make the build slower.

Kotlin is great, but only when your whole project is 100% written in Kotlin.