Agree and I heard Tony Dungy talking about this on PFT with Florio. He said that it's really difficult for a HC who is also involved in the play calling to focus on other aspects of the team, especially during the season when you have few days between games. The offensive play calling HC has his head wrapped around film, the playbook, his offensive personnel, etc. In this modern day of football, with all of the analytics and sheer volume of information to process, I would think that a HC that tries to focus on one side of the ball too much, could leave to much to chance on the other side of the ball, as well as Special Teams. To me a great HC is a guy who can be great at delegating all those duties to other coaches, but still have a solid enough understanding of each working part to stay involved just enough, to make sure things are running smooth. I'm sure there has been one, but I don't remember a season under McCarthy when the special teams was very good. The defense had a year or two of quality play/players, but other than that, its been pretty Meh too.
I am sure at least one poster will now tell me about all the great HC's that also do all the play calling, cause I am sure they are out there. But I would rather see the Packers hire a solid OC and let him run the offense, while the HC coaches the whole team.