I've always believed that except in rare cases where a coach exposes his QB to a severe injury, a coach cannot "ruin" a QB as many here believe. More likely is the QB ruins the coach with his bad play. Most QB busts are the result of bad scouting, not bad coaching. Anyone who thinks better coaching would have worked with JP Losman or EJ Manuel is excusing the people who scouted and drafted them.
That said, there's a relatively new phenomenon that used to never happen: Early round draft choices who were basically on the scrap heap & considered busts are turning around their careers & becoming productive starters. They'll never reach Mahomes/Allen status, but they do recover later in their careers. The 1st was Geno Smith, followed by Baker Mayfield & Sam Darnold. Ironically, each awakened on his 4th team. So, while some would say they were ruined early on, none of these 3 QBs were permanently ruined. At best you can say they were stymied by bad coaching, but ruined, no.