It's a pity its adoption is stagnant. But it was highly successful in at least two respects:

* introducing Java developers to the world of FP about ten years ago (i.e. before JDK8 and Kotlin) with a lot of that knowledge transferable to Kotlin in particular

* significantly influencing Kotlin and evolution of Java itself

Don't forget that it also made Spark (and to a lesser degree Flink and Kafka) possible.