Since open source is practically standard in programming language design nowdays, it is almost definitely Apple's intentional choice to make Swift closed. Also, they could take an existing open source language that is in the same category (OCaml(F#), Rust, Nimrod, Haxe ?). Being closed, prevents developers from cross compiling Swift for Android, and it will make almost impossible to write an alternative Swift compiler from scratch, since the language is not standatdized.
