> Java libraries contain methods that return existential types (e.g. List<?>) everywhere.
I don't know what libraries you are looking at but these are extremely rare these days. At least for the popular ones. You seem to be talking about Java like it was written eight years ago.
I gave List<?> as a simple example; usually we're talking about List<? extends Foo> or similar. Anything that involves a ? anywhere is an existential type, and they're the only way to write a whole lot of things safely, e.g. any kind of callback has to involve a "? super" type to avoid disallowing perfectly valid code. Unless the Java community has abandoned generics and decided to go back to using casts everywhere there's really no alternative in the language.
So do you really think Kotlin is used 4x as much as Java? Or is it possible that this is a pretty poor measure of actual usage?
> As for your other point, tying Java interop with existential types is pretty absurd.
Only as absurd as Java. Java libraries contain methods that return existential types (e.g. List<?>) everywhere. If you can't represent that type in your type system (note that it's not the same type as List<Object>), you can't have full Java interop.