[dkarl]

A language doesn't just keep getting better and better with age. "Maturity" is about achieving a a level of stability and quality in language features, tooling, runtime, library ecosystem, etc. Scala and Java share the same runtime and library ecosystem, and Scala is arguably more mature in its language features, since Java has been forced to add features (and incur extra complexity) playing catch-up to newer JVM languages (including Scala.)

Both languages actually suffer from maturity in their tooling, because their standard build and dependency management tools (maven and sbt) are outdated and crufty, while newer languages such as Go and Rust have tooling that was built more recently, with the benefit of more recent experience.

[kagakuninja]

One of the strengths of Scala is also a weakness. They aren't afraid to break things and make changes that are not backwards compatible. The Scala devs have been much more careful about breakage than in the past, so I think they have reached a good compromise between stability and evolution. The masterful execution of the Scala 3 upgrade is an example.

Java by contrast will never clean up the bad, inconsistent or obsolete cruft in the language. If it is really important for you to run ancient JARs on Java 21, then the Java approach is superior.