Java has been catching up on language features but I'm not sure if I'm ready to switch back to Java for Spring projects yet. Mostly because of nullability, extension methods and coroutines in Kotlin.

I give you nullability, however extension methods without proper typeclasses is just syntactic sugar, and coroutines will be superseded by Loom.

Java hasn't only been catching up, it now supports pattern matching which Kotlin still lacks.

Especially with the heavy, under-the-hood developments of the JVM, like Loom and Valhalla.

That said you don't need to understand that much to use high-level libraries in the Typelevel ecosystem. And when I do need to reach for low-level constructs, I find nice that everything is built upon clean and composable abstractions.

You mention Akka, it's kind of its own beast. The inner workings go definitely way beyond "better Java", yet Lightbend needs to provide a surface API that plays nice with Java interop. It's not an ecosystem easy to navigate in my opinion.

Scala has been declining steadily for a decade now, and Scala 3 is going to precipitate this state of affairs by splitting the shrinking community even further.

Kotlin, on the other hand, has done nothing but grow for the past five years, no doubt helped by Google's formidable support for it. I just took a quick look at one of its Slack channels, "getting-started", and it has over 38,000 people on it. And that's just one channel among dozens more.

I don't think Java's latest additions are anywhere near threatening Kotlin's rise to dominance since Java is moving at a very slow pace, by design, while Kotlin supports some features and design directions that will never happen in Java.

Yeah right they had to reboot it.

Google is Kotlin's godfather, naturally they are using it, just like Amazon with their own Android flavour.