I wonder if Java's speeding up of new language features over the past several years, after many years of a stagnant feature set, is a direct result of pressure from the likes of kotlin.
Even considering ART as the KVM, isn't a safe deal, as it is written in a mix of Android Java and C++.
And currently its potential is very tiny when measuring all JVM guest languages against Java, using any recent language report.
And if you also consider the upgrade treadmill, and where the average Java developer is relative to say ten years ago, it may turn out that at any given point in time, the Kotlin version you'd be able to deploy may have a number of features the most recent Java version you could deploy does not have.
(to say nothing of companies where 'the version' is decided by committee or fiat and thus having a more obscure language sometimes gets you less scrutiny).
There is nothing to fear from Kotlin, it is a guest language on the JVM.
It is only relevant on Android, as Google is making ART into the KVM.
Outside Android, Kotlin market share adoption is very low, as any trends chart will easily prove.