The Java interop argument regarding union types doesn’t seem consistent with coroutine design choices. You pretty much need a third party library for Kotlin coroutine features to be usable from Java code. Seems to me that there are many potential union type designs that wouldn’t be nearly as messy to call from Java as suspending code is with no additional libraries. In my opinion, it’s ok to require a third party library on the Java end to make calling some Kotlin features clean as long the Kotlin team provides this support.
Although some of the other arguments about maintainability and readability do seem valid. I don’t think simply allowing | operator in between types anywhere in the codebase is the right choice as I think it will be messy and overused.