FWIW I'm not saying Johnson shouldn't be interviewed or even hired. I can see why a GM or owner might think, "This guy got more out of Goff than Sean McVay." If McVay is your point of comparison, and Johnson compares favorably to him, I can see why you'd roll the dice, knowing that this is the only way to pry Johnson from Detroit.
Sheila (probably under the influence of Spielman) took a different approach. She didn't hire a brainy coordinator to be head coach. Instead she hired someone with the personality and experience to lead men and represent the franchise.
Maybe Johnson has that. Maybe he can develop it over time. And if he gets hired away I hope he succeeds.
Anyway, even if he gets a job as an NFL head coach and fails at it, the going rate for a first time head coach is around four years/$5 million per, so he would walk away from his "failure" with $20 million.