Mark,
An excellent post.
People step into leadership roles and to a large extent the success of the class comes down to the energy injected into the class by the leaders. While it varies from person to person each leader eventually runs out of energy needed to continue to grow and maintain the class and the time comes to step aside. When they can carry the load no longer and the people who have been reaping the benefits of their labours do not step forward it all falls down.
It is my view that the success of a class largely depends on the succession of energetic and enthusiastic leaders that can build and nurture a sense of community within the class.

Just the way I see it.

Regards,
Phill