It's standard right across the entire game, in every league, in every country, in every continent at every club. Only a minuscule percentage have stuck with their manager. Average length of service is 18 months.In no other industry would a person in Levy's position have been able to appoint so many under achievers.
Poch is currently the 13th longest standing manager covering all professional English football clubs, 4th longest in the PL, with only Wenger, Howe, Dyche & Hughes managing at the same club longer. By the beginning of next season at least two of those names will be replaced, wouldn't surprise me one bit if at the start of next season Poch will be the longest serving manager.