1) it is my understanding that libraries still need to be in obj-c for maximal compatibility. That is, there are some limitations to Swift code usage from obj-c

1) it is my understanding that libraries still need to be in obj-c for maximal compatibility. That is, there are some limitations to Swift code usage from obj-c. If I'm wrong about this, this would be excellent news! 2) code rot. As new feature development is started, if all the Devs are more comfortable with Swift, it is practical that it should be done in Swift. The obj-c libraries will languish. This is just a statement that if Swift becomes the default language, then Apple should only build new features in Swift. You're correct that code doesn't need to be replaced... if it ain't broke don't fix it, though in my experience eventually someone will find a way to break it, and it's at that point that someone ideally replaces it.