Craig Federighi (Apple's SVP of software) discussed this topic on this episode of the talk show (https://daringfireball.net/thetalkshow/2015/12/14/ep-139) a few weeks ago.

IIRC many teams within apple are using swift as much as they can, either for app/framework code, or test code. one example he gave was that the dock in El Capitan had been rewritten in swift. He also mentioned that the lack of 32 bit support for swift was a major pain point blocking more widespread adoption internally due to many frameworks still needing to support 32 bit macs.

Reading between the lines, it wouldn't surprise me if 32 bit support is dropped with the next version of OSX to allow more teams with Apple to start using no swift.