And presumably they had a reason for wanting to do so other than Swift Rulz.

What I'm arguing against is not the idea that new code or new versions of things shouldn't be in Swift. Maybe they should, maybe they would be better in Obj-C on a case by case basis. I'm arguing against the idea that all the existing Obj-C code is an embarrassing legacy that must be purged as a matter of urgency or else we should all start panicking about it.

Heck, large scads of the OS and the application services frameworks are written in plain old C - HealthKit is plain C. Long may that continue. Same for Obj-C.

There is a thread on the old Apple Swift forum that addresses why they like Swift. Its an old post from 2014 called : Why Swift? And why now?. You will need at least a free developer account to be able to log in and see it.

https://devforums.apple.com/message/986257#986257

"13 of the security issues fixed in 10.9.4 could have been prevented or mitigated by Swift (9 buffer overruns, 2 int under/overflows, 2 null derefs). That alone makes it worth it from my point of view.

As for performance, that's a work in progress but is improving rapidly; Swift is already faster than ObjC in some cases."