Besides Spark, is there still a niche that Scala fills? Why wouldn’t you just use Kotlin or Java 11? I guess I should ask the inverse question too: why wouldn’t you just use Scala for everything?
I'd use Scala or Rust for everything.
In Java, one creates a 55 lines getter-setter class with concurrency bugs and accidental mutation risks, and maybe plus a 55 lines Builder too.
Whilst in Scala I create a 5 lines case class, immutable and thread safe, all that's needed.
Java is getting data classes (case classes in Scala), as well as value types.
Data classes are nice, but they are not a general substitution for Properties, which address the greater getter/setter boilerplate concern. That said, you can use project Lombok for that pretty straightforwardly. You can also use project Manifold[1] to add useful features many of which Scala lacks:
* Type-safe metaprogramming (like F# type providers)
* Structural Typing
* Extension methods
* String interpolation
* Templates
* Type-safe reflection
* [Un]Checked exceptions