Instead of blaming coaches for ruining QBs, blame the scouts & GMs who over-drafted them. It's amazing how many people think numerous bad QBs were ruined when they never had NFL talent to begin with. A lot of QBs just can't play at the next level, but GMs & scouts miss what becomes obvious once the guy goes up against grown men in the NFL. Then when the coach sees how bad they are in practice & games and bench them so they won't lose the locker room, the fans and media cry that the QB is being ruined, when in reality, he is a bust. The 1st ones to notice that are the coaches who see them about 5 days a week during the season. Then there are the QB busts with overinflated opinions of themselves who get drafted way too high. Best examples are lazy bums/head cases like Leaf, Russell, Manziel, and Rosen. They never made a true effort to improve their games. Then there are the undersized guys like Young & C.Williams who get drafted #1 and are quickly surpassed by the normal sized QBs taken after them.
PS: Good examples of bad scouting & GMs: The Bills drafting JP Losman & EJ Manuel. Both got benched early in their careers by coaches who had seen enough to say No Mas.