OK, so Sing installed discipline. I think Vernon would have found his way anyway, but maybe not. Not sure Sing helped PW; in fact, PW is having a better career than Sing had. Taylor Mays is an excellent example of Sing's acumen. I doubt Sing will ever get a chance to head coach again.
Nolan did improve the D. He does that everywhere. But he moved them from horrible to average, and could not get them to progress from there. Neither he or Sing did anything for the O.
IMHO the worst of all was Erikson. He got dealt a bad hand because the team was gutted by the front office, and Donahue was perhaps the worst GM in the history of the NFL. But just try to name any one good thing Erikson ever did.