Modern football is an unforgiving sport. It used to be widely accepted that three seasons were required for any manager to put his own mark on a team. In the whole of the 1980s, Manchester United, Arsenal and Liverpool had just three managers each. Leicester City, Tottenham and Chelsea had four. Manchester City had seven, and another seven in the 1990s, but that's the sort of club they were in those days, and frankly, football is all the poorer now that they're so much richer.
Generally speaking, there was an understanding that time was required for change to be effected.