A great QB often makes a 'great' coach, the opposite usually isn't true but truly great coaches can usually find an above-average QB within a few seasons.
The most interesting situation, to me, is in NY. Miami had Marino for about 18 seasons, before that Griese. They've developed some marginal talent--Fielder and Tannehill--they've just never hit on the 'one'. The Bills had Kelly for over a decade, before him Ferguson was at least stability even if he wasn't good (I was too young/not born during his time, but at least going by team record it doesn't appear he was anything special). Then there's NY. Going all the way back to Namath they've bounced from QB to QB, the longest stretches were by immortals like Ken O'Brien and Rich Todd. We're talking forty years since Namath, and they haven't been able to hit on a single above-average QB in that entire time. It's mind-boggling, really.