We had the same hope for Scala - the idea being that the developers who were interested in using it functionally could do so in the more complicated parts of our application, but the 'rest' of the developers, many who were honestly either not capable or just not interested in picking it up, would be able to still use traditional Java.
Unfortunately, Scala just had too many incompatibilities: things like Options instead of null or to call a Scala method in Java would require some confusing method call like new Function0<String> because that is how Scala converts to bytecode.
And collections and immutable types never really worked back and forth very well.
The final straw was when we had Thrift interfaces that simply broke if a project moved from Scala 2.10 to 2.11 without recompiling all the dependencies.
But if Kotlin really is completely compatible with Java without too many show stoppers dragging down productivity, this could be helpful for a lot of larger enterprise shops which contain a small subset of engineers who want and can use the language to improve things (or simply don't want to work with old Java code and will jump ship if forced to do so), but still need to keep things simple enough so that the rest of the team can still be productive.