Traits vs generics: yeah, personally I hope to one day see a language that allows application developers to decide what gets monomorphized, rather than library

Traits vs generics: yeah, personally I hope to one day see a language that allows application developers to decide what gets monomorphized, rather than library developers. It seems like you could usually just synthesize the boxing needed. So why would you ever leave that choice in the library's hands, which has no idea how it will be used?