I think all managers have a shelf life. Eventually it must get almost impossible to get the same group of players to perform to the max. Rich clubs get round this by changing players. Poor clubs settle for where they are and cut their cloth accordingly. Clubs with ambition but not much spare cash change manager. You can do it any time,  it costs less than changing 4 or more players, and the potential rewards are high. Mind you, so is the potential risk so you, as a club and as fans, have to be prepared to fail if you want to get 9n.