As it was mentioned before, Java interop is important for us (we sit on top of a big mixed language code base).
There are some Kotlin-only features (e.g., inline methods with reified type parameters),
but they provide at least some safety guarantees (you can't invoke such methods from Java and expect it to work).
We need strong enough motivation to introduce another Kotlin-only feature, especially one that doesn't provide any safety guarantees.