> Scala killer feature is the assurance that developers can express the things they want
If you mean "some developers" or alternatively "some of the things they want" (this set of things being strictly greater than the set of things available in Java), I'll agree with you.
If you spent a few minutes actually using Scala instead of trying to bash and hate it in every thread, you'd know that Scala doesn't use HM.
Scala killer feature is the assurance that developers can express the things they want, unlike Java and C# where the compiler starts acting like an angry child as soon as I want to do anything slightly more elaborate.
That won't be changing anytime soon.
> a far simpler (though less expressive, even though that hasn't been proven to be a major problem) type system
The lack of typeclasses is a non-major problem? I would say it is an absolute dealbreaker.
Anyway, in what sense it is "far simpler"? Most of the complexity comes from dealing with Java's broken ideas. If Kotlin wants interop, it can't ignore that.