He's a special teams source coach as well if memory serves. Which means he doesn't really lock down one side of the ball for you, as teams in this era prefer due to poaching. That said if you're good at oversight you can get the most out of both coordinators and clearly he's a quality coach. Not up there with McVay or anything, but he's a top half coach who isn't really seen as an innovator.
Also age is a real factor with coaches. There are outliers, but it's a tough job man. I am sure it wears you down. So most of the great coaches even hit a point where the give-a-fuck drops out or whatever else with age and they end up being a shadow of who they were. Which in his case isn't the greatest thing since sliced bread anyway.
All that aside look at the Raiders or the Browns and their chain of hires and a guy like Harbaugh I'd imagine would be attractive. Because he's a known quantity. But yeah I do wonder how much he has left in the tank.