Do you think coaches have anything to do with QB's turning into franchise status?
Maybe. But stability seems to mean just as much.
In the careers of all great coach-QB combinations the debate can always rage about who made who or which was more important.
Many debate what Brady would be without Belichick. Same with Montana without Walsh.
People don't seem to discuss that with other quarterbacks: Manning without Dungy? Still likely very good. Marino without Shula? Elway became an almost black/white quarterback between his years under Dan Reeves and then without but still successful at all times.
I also think coaches who come from offensive backgrounds and/or groom non-1st rounders into stars seem to get more credit in this debate. This defines Walsh turning the undersized and average armed Montana into the leader of an offensive juggernaut during the 80's. Or Belichick finding a system with endless coordinators that somehow turned that pale, flabby, average armed Brady that showed up at the combine that no one wanted to draft (included the Steelers) into a hall-of-famer. Even Sean Payton seemed to hold the "guru" title a bit during New Orleans success because 2nd rounder Brees was even noticeably better (and statistically more dominant) than he ever was in San Diego.
For the most part, historically top-10 QB's seem to develop independently of their coaches (at least in the minds of fans). There is a certain level of expectation that comes with that talent that coaches can only hurt, not help (though we know that's not the case).
As for Roethlisberger, his success and talent manifested itself with Cowher (and Whisenhunt) almost immediately. If anything I think there was significant fan concern that the Tomlin/Arians era actually hurt Roethlisberger's development, pandering to his youthful wishes a bit too much and failing to address or correct the problems of his play style. For many (me included) the comfort level with Arians was a bit too high. I think it took a more forced working relationship with Haley to partly mature Roethlisberger into a better professional. Could he have done that at age 25? Probably not... who knows. And one of the main reasons to promote Arians in 2007 was to smooth over the waters with a new, young, brash Tomlin at the start and to guarantee Roethlisberger would sign that important second contract at the right price. The phrase happy wife, happy life certainly applies to franchise quarterbacks as well.
It is a complex relationship and one the fans don't fully get to see behind the curtain. Becoming great in any profession often takes a little bit of luck to go along with the talent. Being in the right place at the right time. Hitching on to a wagon of other successful and talented people. Understanding the direction of the "success river" and not fighting against the current.
For many talented individuals, both in sports and life, many are caught up losing out in one of the above only to become footnotes of history while others with equal talent become rich and famous for generations. If it was easy to identify or easy to spot the difference and causes, it would be a much different world.