Kotlin is really very similar to C#. Java developers shouldn't have much of a problem either (now that Java finally has lambdas), and I hear Scala is mostly a superset of Kotlin.

GraphQL and (Kotlin or C# or Java or Scala) probably isn't that small of a niche.

Edit: Also, Kotlin does have near total compatibility with Java and can use any Java library (aside from the most magical bytecode-rewriting annotation processors) so I don't understand what you're saying about having to rewrite things that exist.

I'd put it the other way around, Kotlin is a subset of Scala that the masses might be ready for. I would always prefer the kind of candidate who's excited about stuff like Kotlin over others.

The nice thing about Kotlin is that you can just bind to the existing Java GraphQL libs. Clojure is similar in this respect. Sure it means you have to also be familiar with Java and the Java ecosystem, but it's actually great to interact with Java-the-ecosystem when you don't have to write any java. ^^

Why are people even hiring specifically for such a niche subset? These are not huge complex systems, anyone can become familiar with GraphQL in days.