In a sense, Java adoption of a language feature is the ultimate validation for said feature.
This is something I really like about the recent direction of JVM languages. It takes a bit for useful things to make their way into Java itself, but by the time they do they're well-shaken and we can see how they got used in practice.
Algebraic data types has been available in various programming languages for more than 40 years.
Sure, sure. But the point here is that there are JVM languages that have shown that these things can be done well on the JVM.
And Scala even uses the same keyword "sealed". Glad they took that on the Java side, and didn't change it just for change's sake. Maybe there are other languages that had a sealed keyword but again, with Java, they are clearly accepting as convention some of the things that Scala established, which is a good thing.