I believe there are no plans to port Kotlin’s null safety to Java, which is possibly the biggest advantage Kotlin has right now. Once you’ve tried it there is no way you would want to live without it.
In order to replace Java and it's ecosystem one would need to offer MUCH MORE than just null safety...
But certainly if I were to add new features to an existing Java codebase I’d use Kotlin. The interop between the two is fantastic and for me the null safety alone makes it worthwhile.