Pretty much none of Apple's current software is written in Swift though, from what I can tell.
The ability to run 32-bit applications means that preexisting libraries cannot incorporate Swift code yet.
The "application may slow down your phone" warnings that users are getting with 32-bit apps this year is a pretty strong indicator that Apple is going to remove support for running 32-bit apps completely for iOS 11 or 12. They previously had a deadline for apps to have 64-bit version submitted, but backed off the ultimatum for now.
Apps have had to support 64-bit since June 2015 (February 2015 for new apps), and Apple hasn't backed off that deadline. But there are still 32-bit apps on the store that haven't received an 64-bit update, and I don't think Apple has ever stated what it is going to do about them (other than showing the warning).
Apple can't write Swift libraries (at least not ones that are publicly exposed) until the ABI stabilizes, so that will not happen until at least Swift 4. Apple can write Swift apps as long as they don't need to support 32-bit Mac, which they haven't needed to do for years.
There is a WWDC session talking about it.
I would surprised if they actually did tho. I know someone did some static analysis of the apps for mac and iOS and found Swift was barely used at all.
I have to stomp on coreaudiod a few times a month because my USB DAC stops responding.