The long term is always through the Sporting Director. Arnesen left, Comolli's head coach appointment was a failure and he had to go but both left considerable legacies taken on by those that followed. This is clearly why Levy was eager to return to it. No doubt he also compared the vast profits made from players sales under that system with the poor value brought in under Harry.
As for coaches/managers, it should be clear by now that anyone half decent hits the ground running. If they're not delivering clear objective progress after two years.. get rid! All the stuff about replacing coaches = years of transition is guff. See Rodgers, Martinez, Pardew.. did Liverpool, Everton and Saudi Sportswashing Machine slide down the table or take years to recover from the change? No, quite the opposite.
Sherwood starting well bodes well for the long term. The ODVDG started with a team that had finished 4th and left us with a demoralised team in 7th and he was meant to be some sort of progressive coaching genius. And there is no evidence that he'll 'learn' as it is hugely about character and personality. If that isn't there then you lose the players every time. This was the folly of saying Hoddle will somehow have 'improved' from 7 years ago. He had about 15 years in management and was still having the same issues at the end. The ODVDG probably would have failed long term at Porto too.