Why would you convert the entire project to Kotlin? It’s trivial to mix the two languages, and in fact most Kotlin applications depend on Java libraries.
I don’t think that’s true. One of the biggest use cases for Kotlin is Android development. Interop between Kotlin, Java libraries, and the Android SDK is completely seamless.
It's hard to be certain but we have seen this story in Scala. Among the advantages advertised early was interoperability with Java and access to its huge ecosystem. In practice the Scala ecosystem is completely different (well, Akka is a special case popular in Java projects too).
As a trivial example people would rather use ScalaPB instead of officially supported Java library. When I worked on a Kotlin-based system people were adamant to use github.com/JetBrains/Exposed. Which had a couple pages of documentation and was missing features.
And then again there's politics. Big/successful companies always end up re-inventing PL wheels. The whole GOOG/ORCL mess gave GOOG incentive to fragment the Java ecosystem too.
Ok that’s Scala… Kotlin is already the primary language for all new Android applications. The interop story with Java is unambiguous: Kotlin works well with Java. For instance, Android didn’t rewrite most of their SDK in Kotlin, yet it works just fine. Popular Java libraries like Spring and Dagger work with Kotlin. It really “just works”.
