The problem with Kotlin is that Java copies its features pretty quickly. So as time progresses, the benefit gap between the two languages diminishes, and eventually will not justify switching ecosystems. For example, Project Loom is the answer to coroutines. Sealed classes, etc. Up until recently, Kotlin was not a 1st class supported language for Bazel, gRPC, etc.

In addition to Loom being superior to async/await, Java's pattern matching is shaping up to be better than Kotlin's, and Java's string templates (https://openjdk.org/jeps/430) are also superior to Kotlin's.

Superior how?

I'm fine with Java copying features. I'm happy to use Kotlin now while it is ahead, no problem with switching back to Java once it catches up.

This approach breaks down when thinking about teams in a large organization. Language switches are a big deal.

The time for this to happen though is a long cycle though since Java holds such heavy backwards compatibility though right?