If you can have an effective unit only when you have the best players, you aren't a very good coordinator. By effective, I mean middle of the league.
A good coordinator is one who adapts to the players he has and and finds ways to make them effective even if they aren't a "best in the league" type of unit.
Packer defenses have not been merely ineffective, often they have been awful. A good coordinator avoids being awful even with a lot of mediocre players. Face it, due to roster turnover, injuries and what have you, the Packers have had poor O-lines at times, poor running backs and inexperienced WRs at times. Yet, their offense has found ways to remain effective.
I was among those who were concerned with Capers' history. He seemed to field top 10 type defenses for 2-3 years wherever he went, then faded badly. With all the film available to NFL teams, I'm not sure why that would be, but it is.