For me, if Kotlin is on the table I might as just use C# where the entire ecosystem is built around these concepts.
I wouldn’t really leave behind the JVM ecosystem, which is much bigger and better quality than the .NET one from what I gathered (though it is still good and bigger than most other languages’). And other people’s code is the numero uno productivity boost, so it’s an important area.
I think I would be fine with a smaller ecosystem. I've seen a lot of projects that have made bad choices in libraries because there are too many. Sometimes I feel like if there was just one blessed jdk way it would be a lot simpler. Honestly, the only thing that is keeping me from .NET right now is async/await. Pron really did Java a service by pushing for virtual threads.
I might want a nearly-Java language that only adds trivial syntax sugar, but are otherwise the same. Groovy sort of had a good initial idea here (being a superset of Java, though again, it has grown apart a bit since).
For me Kotlin lives in this weird limbo between these two extremes. If I want a different language on the JVM I would rather do Scala or Clojure.