> I've worked both at a startup that used functional programming libraries pretty heavily, and a startup that is using as plain of a Scala as you can imagine (no cats / zios / kyos / etc. in sight), and both benefited greatly from Scala language itself.
Wouldn't you want to use Kotlin for the latter these days? It seems to have broadly wider adoption than any "plain" variety of Scala.
I don't know, kotlin has its pros, and is absolutely here to stay due to Android alone, but I always felt it being more or less just syntactic sugar for Java 8, to the point that now even Java has some more advanced/elegant features, like proper pattern matching with algebraic data types, while Kotlin is stuck with dumb sugar for instanceof checks.
Scala was always much more elegant and actually going the extra mile.
Kotlin's adoption is not that much bigger than Scala, and it's no good to me, as it's heavily concentrated in the Android world, which is irrelevant to me. Among other things, that means that Kotlin.js is not as good as Scala.js, both itself and in terms of the ecosystem – and the quality of Scala.js is half the reason I'm using Scala in the first place.
Kotlin as a language is also closer to Java on the Haskell <---> Scala <---> Java spectrum. We just don't want to move in either direction on that spectrum, Scala sits perfectly where it makes sense to us.
It's interesting that some people here are saying that Java's recent improvements will eat into Scala's market, when actually, it seems that it will eat into Kotlin first.
Wouldn't you want to use Kotlin for the latter these days? It seems to have broadly wider adoption than any "plain" variety of Scala.