> But somehow Kotlin managed to stay "not too complex", unlike Scala.
It's not really true anymore, Kotlin has slowly absorbed most of the same features and ideas even though they're sometimes pretty half-baked, and it's even less principled than the current Scala ecosystem. JetBrains also wants to make Kotlin target every platform under the sun.
At this point, the only notable difference are HKTs and Scala's metaprogramming abilities. Kotlin stuck to a compiler plugin exposing a standard interface (kotlinx.serialization) for compile-time codegen. Scala can do things like deriving an HTTP client from an OpenAPI specification on the fly, by the LSP backend.
> JetBrains also wants to make Kotlin target every platform under the sun
So did Scala long before. It's just that Kotlin got a lot more traction for different reasons.
Not to the same extent. Scala.JS and Kotlin.JS are somewhat comparable, other targets not so much. There was no serious attempt at making Scala target mobile devices, even during the window of opportunity with Scala on Android.
> even during the window of opportunity with Scala on Android.
I don't understand this. You can run any pure Java jar on Android, pretty sure you can do that with Scala too? It's not exactly a "different platform" in terms of programming language. Sure it needs tooling and specific libraries, but that's higher level than the programming language.
Jetbrains is doing interop with Swift (Kotlin -> ObjC -> Swift and more recently Kotlin -> C -> Swift), which Scala never did. But I don't really see how this is relevant in this conversation.