I beleive that Kotlin was built not to sell IntelliJ, but to build IntelliJ. JetBrains has that line of about 20 IDEs all written in Java sharing an awful lots of common code. I think that together it is among largest Java codebases still in super-active development.

So, you have to code on Java 40 hrs per week year by year. All those `Foo foo = new Foo();`, every day. And you think: "If only there was a language like Java, but better, and it was interoped with Java, so you can gradually introduce it into Java codebase!".