I don't think there's a problem with hiring a former DC if he also understands the evolution of offense (see Nick Saban). The problem with Pruitt is he's wedded to an outdated 1990's offensive philosophy (as is Fulmer). I don't think Fickell suffers from the same affliction.
Plenty of great OCs who have flopped or been mediocre, as well. Gus Malzahn was one of the greatest offensive innovators of last decade, but he was only so-so as a head coach; supposedly he wasn't great at player development. Of course, that's why it's so difficult to hire; it's tough to figure out how skills translate. Some people are great at X's and O's, but bad at player development or managing a team. Some people are good at player development, but awful at hiring. Some people are great with development and recruiting, but awful at management.
It truly is a CEO type position. You have to be good at a lot of different things and even more importantly, you have to know what you're not good at, and find people who can make up for your weaknesses. I think that's what Saban does better than anyone. He knows where he's weak and hires the right people to offset that.