New versions don’t have breaking changes, they are just not binary compatible between major version upgrades which is true of many other languages as well. Regarding complexity - scala has many features due to having a small number of clever primitives. Kotlin special cases many common cases, but imo that will be a more complex language down the line. I really don’t know what do you mean by academic features — one can make haskell libs with scala but they are libs, not language features. And build time-wise it is not that much slower than kotlin.