> And? Nothing "needs" to replace the language, though

I think this is a really important thing about JVM langauges. They can exist successfully at much lower thresholds of popularity because they piggy back on Java. So I don't have to worry about Groovy dying off (which is not to say it is) - because basically as long as Java is around it is going to have access to a giant community of developers. So using Groovy or Clojure or Scala etc is a much lower risk proposition than jumping in the deep end with a stand alone language. In that sense, Groovy / Clojure / Scala / Kotlin don't really compete with each other or Java as much as help each other, and Java's popularity is really the sum of all of them.