> And if you are starting a new project, why would you pick Java over Kotlin?

I've written multiple production services in Kotlin Spring Boot. Now, we're building a new system and using Java 21 (25 soon).

Why? Kotlin the language is great, but there are corresponding tradeoffs in interop. Meanwhile, Java the language has improved to the point that it's good enough, and Java feels like it's headed in the right direction. In my opinion, AI models are better at Java than Kotlin. If you prefer a weaker claim, the models are trained on more Java code than Kotlin code.

Finally, from an enterprise perspective, it is a safer long-term investment for a Java shop to own an application written in Java rather than in Kotlin.