A head coach has three basic jobs: create the culture of the team by motivating/disciplining player, selecting top coaching staff (OC, DC, STC, etc.) and top level game management (timeouts, challenges, etc.).
Yes there are head coaches who are also their own OC/DCs (ex. Payton, Ryan), but the vast majority follow this model. Note I do not include personnel selection through the draft, because on most teams the head coach is just one vote among many, and generally not even the biggest one.
That said, I challenge you to find another coach in the league who crafts and preserves team culture better then John Harbaugh. Whatever the talent level of his teams they play their asses off no matter how far in the hole they are and that is by no means a given even in the NFL (see Browns, Jaguars and other bottom feeders).
Can you question his performance in the other two areas? Maybe, but pop over to any team with a "top 10" coach and you'll see them bitching just as hard as we are (over at the Steeler's forums they have this same discussion after practically every loss despite being a perennial play off team just like us).
The chances of randomly promoting a college coach, OC or DC and expecting John Harbaugh or better is laughably small. The new ones are a crap shoot (HC is NOTHING like OC and DC) and the old ones usually have at least one issue or they would still be HCs. Firing an HC is something a club goes to great lengths to avoid because it can trash your franchise just as fast as a bad QB pick. Plus, do it too many times (like the Redskins) and the best candidates wont even apply.
We just drafted high for the first time in nearly a decade and by all accounts did a spectacular at it, if it happens again at the end of this year it'll suck but I expect a similar influx of talent. 2018 is the earliest I expect a serious conversation about replacing Harbaugh given the team's current trajectory.