I've heard it was more that he was tired of upper management trying to meddle in his coaching and whatnot. Dude was in the middle of making a dominant team and was fighting with Baalke over stupid shit it sounds like.Harbaugh definitely wore out his welcome at SF. At basically every level too. Players got tired of the corny act, management got tired of his antics. He went back to the college level for a reason.
Hell, Even with Pete Carroll, probably one of the most likeable coaches in the NFL, it's really started to seem like players aren't buying in to his thing much these days either. So, it's something that's just gonna happen when egos are involved.
Harbaugh would be my first choice. in a distant 2nd, I'd kinda like to see what Ben Johnson could do. Pretty "meh" on rolling the dice on Quinn.