I've seen this many times in 20 years of coaching. Some guys are great position coaches and can't quite step up to be good coordinators (Pellum at UO?), and even many great high school coordinators are lousy head coaches. So much more is involved in the HC job -- so much of it is leading players and staff, working with admin, parents, boosters, off-season programs, feeder programs, managing staff, etc. etc. Plus the on-field leadership and coordination. Motivating players and keeping them out of trouble. I have great respect for guys who can do it -- there is just so much to the job. Plus you need tenacity, thick skin, and tremendous physical, mental and emotional endurance and toughness.
I really liked Helfrich as a person and I think he was a very good OC. He didn't quite have all the necessary skills to be a good HC. But there is no shame in that, since it takes an extraordinary man to do it. And he tried. I wish him well and hope he is happy.
Here's hoping Coach Lanning is that extraordinary man, who can lead, manage everything, and stay humble and likeable.