It doesn't matter who you hire, as long as there's actually a resume behind it. Someone with an "I did it in college and hung around an NFL team" resume can work their way in as a position coach like everyone else. You don't bring them on as coordinators, let alone sole OC and QB coach.
"You should never hire the legend player." Yes, you do hire them if they deserve the spot. What they were doesn't play any part in it.