Same experience here. JDK11 and beyond added some more enthusiasm to base Java. If Loom lands I think a lot of people will also lose interest in Kotlin as well (because AIUI a big draw for Kotlin is the async story).
I completely disagree. The best things in Kotlin has nothing to do with coroutines, it is the extension functions and the soon-to-be released typeclasses (multiple receivers). Java is lagging behind, while Kotlin gets pragmatic and useful features very frequently.
I have some friends/colleagues who use kotlin in personal projects and they enjoy it. But I've never heard them talk about those features. And they certainly aren't enjoying it now because of soon-to-be-released features.
But I am interested to learn more. Do you know any good resources?
I think the soon-to-be-released features anxiety is applicable for any language, it is not unique to Kotlin. A good resource for this is the Kotlin Koans for example or the Kotlin STDlib itself. I also maintain some OSS projects that use these things extensively (pun intended). What's also present in the language is the multiplatform functionality. You can't do this in Java and you probably never will be able to do it. With this I think Kotlin is one of the two languages that's capable of isomorphic behavior all across the board (you can write everything in Kotlin).
Most Kotlin devs I know are not particularly interested in coroutines. There is a lot more to like about Kotlin: native function types (instead of functional interfaces), unified type system, bottom type, Unit type (instead of the special case "void"), extension methods, no checked exceptions, sealed classes ...