I rewrote a commerical app to Swift and saw a lot of advantages. The rewrite uncovered a number of bugs due to the stricter type checking, and general strictness (no unitialized variables e.g).

It also allowed us to get more people involved in development. People new to apple development found Objective-C to be a bigger barrier. Odd syntax and a bit old fashion.

Mind you I quite liked Objective-C. But it seems a bit pointless when you got Swift.

dock is one of the oldest apps on macOS, was rewritten in swift when it was announced.