The mark of a great leader is that he or she leaves a company, country or other organization strong enough to stand on its own without their continuing influence or indeed choice of successor. Redundancy is the ultimate test.
Some potentially great leaders do in fact succeed in this respect, yet fail to recognize their own achievement, and thus linger on, often to the detriment of their legacy. For it is true that everyone rises to their level of incompetence. It is no criticism of human ability or human nature, mere a recognition of our limitations.