Kotlin extension methods can't be grouped and abstracted over like traits can. And moreover they are not virtual methods. Virtual methods are useful for all sorts of reasons.
In Kotlin extension methods are best used as a nice way to add utility methods to code you don't directly control, and as part of the 'extension lambda' system that lets you make DSLs. The way Kotlin defines language integration features using methods with magic names makes it easy to add a thin layer on top of a Java API that makes it much nicer to use, without needing to fork or patch the actual library itself.
