Having a little difficulty following this. In the Ticketing function, the #1 guy leaves, and the #2 guy takes over as the interim. So for one year they are down one body/total comp.
Then the #2 guy leaves and they are down two for two or so months. There is no internal candidate to promote, so they hire one from the outside as the #1. Back to being down one body. Has the #2 slot been filled?
The knee-jerk reaction that an open position is automatically filled is that of a bureaucrat. A manager attempting to optimally allocate financial resources looks at it otherwise, particularly if a slot has been open for a year with no deterioration of function.