If someone thinks two years sounds pessimistic, nine of the current PL managers have been at their job for less than a year. Further three for less than two. So only 8 managers have been at their club for more than two years. Further three for less than three years. Only Wenger, Howe, Dyche, Pocchettino and Hughton have been at their club for more than three years. That's exactly one quarter.
Without a DoF picking the manager would also be left to amateurs, like the CEO and board. They pick the next manager who is then given a transfer fund to buy players that happen to fit his philosophy (+ some old players he gets along with).