IMO That would probably put some stress in the compiler since the value of default args can potentially be a function or value that also may use inference. It could also promote bad practices since public APIs should also always be properly typed. Types serve as documentation.
In Scala most people started using inference everywhere and then I think most of the community agrees it was a bad idea and that the more typed the better. Not sure how that will play out in Kotlin land and I'm assuming the lang style guidelines will touch on this subject. Maybe someone from Jetbrains may weight in as to what their recommendation is in Kotlin.