the argument for Java seems to be that it's gradually gaining features we already have in Scala, Kotlin, OCAML, F#, etc... so why not use the languages that have those features today?
Inertia. There are still a ton of Java shops out there, and many of them will not switch even partially to another language anytime soon. The hope is that these orgs might find it easier to upgrade to Java 21+ than to learn and start using Kotlin/Scala/etc. I fully expect I might find myself working at such an org again, and when I do I’ll be grateful for newer Java features.
Signed, a dev who would never willingly choose Java over Kotlin for anything ever again.
One thing about "gradually gaining features" is that the Java language architects do a very good job with backward-compatibility. This means that your application/library will be easier to migrate to newer versions of the language and you will continue to have a large number of 3rd-party libraries available. The Scala community is currently migrating to Scala 3 and many libraries have not been ported (Scala experts correct me if I'm wrong.) Kotlin also seems to be less stable at the API level than Java (but I may be wrong here, Kotlin experts please correct me.)
This backward-compatibility does comes with costs (e.g. non-reified generics)
Also, if you're a library developer and you want to create a JVM library that can be used by Scala, Kotlin, etc. developing in Java is often the best choice. For one thing it avoids any dependencies on the standard libraries of those other languages.
Disclaimer: I like Kotlin and Scala, but mostly use Java.
