I highly doubt it. Even if we accept that Newton is better than Roethlisberger, after Bell went out the Steelers only had Antonio Brown and not much else to threaten the Patriots with. Brown was contained and, with that in mind, I have a feeling that the #1 defense in the NFL would have found a way to rattle Cam and get him to turtle in yet another big game like he did in Super Bowl 50. Further, Cam doesn't play defense. Brady still would have tuned up the Steelers' defense. There's no way they win the Super Bowl just by replacing Ben with Cam because they wouldn't have even made it there anyway.
Reid's failures have been as a head coach. His in-game adjustments have been nothing short of bad at times throughout his coaching career. Due to that alone, those early-2000's Eagles teams underperformed in spite of the fact that they reached all of those championship games and the Super Bowl. But his ability as a quarterbacks coach shouldn't be questioned. He's gotten the most out of Alex Smith and he got the most out of both McNabb and Vick (albeit for one season). Mahomes is in the ultimate situation due to that fact coupled with the fact that he has the opportunity to sit, wait, and learn. This is Smith's team right now and he won't be thrust into action immediately.
Actually, if we're taking away the possibility of a QB pick blowing it because of his own stupidity, I'd say that Kelly is in the best possible situation. IMO, Kelly was the best pure passer to come out in this draft and would have a very bright future if football was his #1 priority. But it isn't. Partying and sliding into Mia Khalifa's DMs are his two top priorities. Unless that changes, he won't have much of a future in the NFL.