One thing I and a lot of people are missing is annotations, with them data classes in Kotlin are useful for JPA/REST/AMQP/etc DTOs. Another convenience would be the ability to add at least mapping methods like in Kotlin or implement interfaces (Serializable anyone?)
Java records are going to be yet another barely used feature.
Record have an automatic way to propagate annotations from the record component to the generated methods/fields and comes with native support of serialization (like enums).
There you can read about algebraic data types in java, in Java 14 you get preview for records in Java 15 probably you'll get preview for sealed types - they were developed together for some time, only recently there was a split.
"Beyond the restrictions above, records behave like normal classes: they can be declared top level or nested, they can be generic, they can implement interfaces, and they are instantiated via the new keyword. The record's body may declare static methods, static fields, static initializers, constructors, instance methods, and nested types. The record, and the individual components in a state description, may be annotated."
Scala 13th, Kotlin 20th.
I expect the current annotation-based libraries will start supporting record classes too.
The survey SureshG put forward (in a post parallel to yours) shows Kotlin twice the size of Scala (which surprised me), and Java something like 15 times the size of Kotlin. So to say that Kotlin is popular, but Scala isn't, seems to me to be drawing the line in a fairly arbitrary place. It seems more reasonable to say that neither is popular, or that both are.
"But Android" you say. Well, that will make Kotlin pretty popular, but it hasn't yet. In the future, you'll be right. That's why I added this. In the future, my post is going to look pretty silly without this context.