It's worked out pretty well for them I'd say. Consider that any developer can write a plugin for it easily, that cannot segfault the entire app, and which runs on all the platforms the IDE supports. That's just one advantage. Higher productivity of feature development is probably another.
To see how things could have been different, look at Visual Studio which is still a 32 bit program even in 2015. Users of it complain that it simply can't load big projects without running out of RAM even if their computer has enough. The VS team says they aren't doing a 64 bit port because it's such a big code base and besides, their plugin model is fully native. And of course it's Windows only.
At the time (~2002?), I think Java was a very reasonable choice for an cross-platform IDE. As more programming language research and different programming paradigms continue to gain mainstream adoption, Java's emphasis on backwards compatibility and stability makes it a bit difficult to change some of the initial design decisions that Java started out. However, since Java is so mature with an even more mature eco-system and an incredible runtime specification (JVM), it made sense to build a modern day "blue collar" language that could be classified as "Java, if it started out today." With that in mind, and some of the same principles that Java was founded on, Kotlin delivers on this promise. IMHO, it is the only JVM language that really does bridge this gap.
They didn't have to - JetBrains has been around for a long time and Kotlin is new. The productivity benefits mostly come from things like fast compile times, garbage collection etc.
To see how things could have been different, look at Visual Studio which is still a 32 bit program even in 2015. Users of it complain that it simply can't load big projects without running out of RAM even if their computer has enough. The VS team says they aren't doing a 64 bit port because it's such a big code base and besides, their plugin model is fully native. And of course it's Windows only.