yes I feel that scala or clojure already offer all these advantages (depending on your preference of static vs dynamic typing) and have great java interop.
I'm not sure, but I know the Gradle people worked with Jetbrains to integrate it into Gradle, so you can use it instead of Groovy. SpringBoot initializer will allow you to choose Kotlin as a language (along with Java and Groovy). Corda (an open source distributed ledger was built with it) - not sure how many are using Corda since it's so new. Quasar (an JVM actor library) supports it. Here's an article about KeepSafe converting their Android app: https://medium.com/keepsafe-engineering/lessons-from-convert.... There are several companies listed on the Kotlin website as well.
I guess my point is you can find people using it. I just wonder why anyone who is currently using Java doesn't at least consider it and try it out -- it's much more concise and nicer to develop in. But then again, I guess a much larger percentage of Java developers don't ever learn anything new and probably have never even heard of the name.
Kotlin's backed by Jetbrains which makes IntelliJ which many Java developers have heard of, whereas Apache Groovy was dropped by Pivotal which also still backs Spring. I imagine more and more software which enables Groovy for scripting will follow in the footsteps of Gradle and SpringBoot by allowing Kotlin to be used as well, such as Jenkins and maybe even Grails one day.
I only know one person at a local meetup who is a fan of the language. He makes a good point that it does generate binaries that work on Android, though.
(since the newer Android SDK is based on Intellij, I guess that's not too surprising, but still useful)