An organization should have a plan to replace everyone in the organization. People quit, die, retire or want to move their career in another direction. It's a manager's job to have plans in place to replace everyone in their organization.
When I first started working (many years ago) the whole concept of succession planning was a large part of a manager's job. I don't think it is done nearly as well (if at all) today. Secrecy on succession planning is important.
While we don't like to think about it, running the athletic department (as well as other departments in the University) is a business and good management principles apply to ND as an organization.