How about looking at the number of people who must be removed in order to bring the bus factor down to 1? We can call it the "bus co-factor" :). With the bus factor we're picking the most critical people first, and removing the least number of them; with the bus co-factor we're picking the least critical people first, and removing the greatest number of them.

There's probably already some name and many theorems in graph theory for both of these ideas.