About one and a half year ago we also decided to give kotlin a shot. After a few months my opinion was similiar to yours. Extension-functions, yeah! Anyway,

About one and a half year ago we also decided to give kotlin a shot. After a few months my opinion was similiar to yours. Extension-functions, yeah! Anyway, nowadays you wouldn't see me making the same statements anymore. My first issue appeared when I was maintaining an Android app written in Java. I needed an library for some functionality. The only maintained ones were kotlin-libs. But interoperability, right? I didn't hesitated to include it. There is no way to use or extend functionality with typealiases in your java code. IntelliJ can compile Kotlin into Java and it is easily accessible. But there is no adb debugger for kotlin. Null Safety is just overhead imho. I know when it is appropriate or neccessary to check for null, otherwise i can get rid of it. And its more easily grep-pable the Java way. Writing Kotlin is not so fluent as one would guess, the tooling only fools you, that it is. You can easily make use of actions like typing pfi (extends to public final int) for java aswell. But when your property changes from nullsafe to nullable, you need to rewrtiging eeevverything. Additionally, i do not like the implicit type on member property declarations used by almost anybody. The benefit of typing less boilerplate becomes annoying when reading sources outside your ide. Boilerplate in my opinion is code i need to write by myself. When it is generated by any IDE, i do not bother about it. We are so familiar with Java that we can abstract this boilerplate when reading sources. Spelling Kotlin to somebody via telephone is also strange for me. I stopped promoting Kotlin as a Java killer. Code style is too different between different developers. And why are we creating the same libraries once again?