I think all of the work groups within the Swift project take the latter approach out of necessity based on who in our community has the demonstrated interest and capacity to serve on the work group. For example, in the recent Swift on Server update, two members of the work group stepped down while three new members joined. The Diversity in Swift work group is modeled after the other work groups here and we intend to operate in a similar manner.
That said, I would be absolutely thrilled if enough individuals in our community do have the interest and capacity to serve on the work group so that we are able to have an entirely new leadership team in two years time! As others have pointed out in this thread, diversity in our leadership is important, and part of that is giving new folks the opportunity to serve as leaders.