Have you ever been at a school that changed for the worse when a principal left? At a job that changed for the worse when a manager/supervisor/boss moved on? One person can make a huge difference!

Things changing for the worse isn't evidence that the person that came before was a bad leader - just proof that it's often difficult to maintain greatness. I think this difficulty is compounded in high-turnover environments (like a football team, with players going and coming from missions, 1/4 of your roster being replaced each year, coaching hirings/firings, etc.).