Hartline just made the most sense. Osu2 was going to be hard pressed to find an OC of Chip's caliber and if Hartline wasn't promoted he would probably be looking elsewhere (Day doesn't need to lose yet another coach right now). He's been there for 7(?) years now so he's put in his time, the players and recruits know him well, and he's probably ready for the next level.
That being said, even in the best scenarios replacing a coordinator is significantly disruptive. DL was really fortunate to snag Will Stein as he is very similar to Dillingham (young, dynamic, creative, similar offensive philosophy). But Day is going from one of the most experienced and successful offensive coaches in Chip to a newbie who has never called plays before. I think Osu2's offense takes a step back next season (and perhaps the season after) as Hartline learns on the job and makes mistakes like every newly promoted young person does. There's a window of opportunity there for OBD and Penn St to overtake the Buckeyes during that span.