When the two arrived they worked alongside each other, one the brains, the other the motivator.
Hignett heads back to familiar parts and the wheels start to fall the wagon.
Cooper ends up down the yellow brick road.
At the time it appeared to some that Hignett's departure was the defining moment and his leaving was the reason things went down the pan.
Maybe they just played different complimentary roles and whoever left there'd have been a problem. So Hignett's return was the return of a coach but not a manger and we still needed a wise head alongside.