Comment by leonatan

There are a lot more challenges than just ABI and 32 bit. Swift is not dynamic, lacks many of the features the ObjC runtime provides - so most of Apple's frameworks cannot actually be implemented in the same way with Swift. Basic stuff like KVC, KVO, responder chain, etc. go out of the window. And many of Apple engineers are not convinced throwing these technologies out of the window for some imaginary benefit from Swift is the right direction.