Interesting point. Usually the vet case is made that the Packers (and esp. Capers) needs vets because of their usual callow and inexperienced yutes. In fact, Le Beau doesn't want young players to have to play in Pitts D.
But I don't think Peppers has exhibited too many problems with his learning curve. I think the issue is mainly finance, risk and downside of careers with FAs. I doubt the Packers system is too complicated that it hampers FAs.
But perhaps the best point is the run D problem itself. Run D is keeping your defense in shape and on target (gap integrity). There are fewer lessens more basic than that in football but somehow the Packers need to relearn it every season. If Capers was being too complicated, it would show up more easily in the backfield.