This.
Hartline seems as in love with OSU and the city of Columbus as Jim Leonhard is with Madison and UW. Leonhard decided to stay WI despite being passed over for the HC job to Fickell and knowing Leonhard could have about any DC job at another school or even in the NFL that comes open this winter.
It's so much harder to get and stick at a HC gig when you're already a coach at that school. Of you are promoted, you get no honeymoon. Results are expected right away if the previous coach was getting it done and retired or Left for a different job. You can't get HC experience while you're a DC or OC, much less a position coach.
The only way to do that is to leave the comfort of the situation and prove your mettle elsewhere. Hartline has to leave to be ready to get the HC position at OSU. It's not optimal because it's not convenient, but it's important if not critical.