On iOS 10.1 there were only 4 binaries using Swift. The number of apps and frameworks using Swift grew quite a lot in a year: There are now 20 apps and frameworks using Swift in iOS 11.1 […]
Similarly the number of binaries using Swift grew from 10 in macOS 10.12.1 to 23 in macOS 10.13.1.
It looks like most of the system components built in Swift are entirely new apps, or effectively so, as with Music and Podcasts. But it also appears that Apple is thoroughly porting both operating systems over to Swift. I have no idea how deep that will run — I imagine device drivers, for example, may not be rewritten — but perhaps the goal is to have everything the user interacts with be built in Swift, or something like that.
Whatever Apple’s specific goal may be, the apps they have ported to Swift so far are not little things or developer-specific utilities. These are critical apps that people use all the time. If that’s not eating your own dog food, I don’t know what is.