It was Thurman Thomas not Kelly that made the difference in those games. Marino was a far superior QB though Kelly was a perfect QB for the K-gun and the Buffalo weather. Marv never outcoached anyone. Marv was the beneficiary of one of the top 5 GMs in the history of the NFL. To Marv's credit he created a culture that was able to overcome the bickering Bills and somehow get a team to battle back year after year. I am still amazed he made the HOF. He was outcoached by Parcells/Belichick, Gibbs, and Johnson.
I don't now what the hell happened to Shula in the 80's. He had the premier QB in the NFL with a superior O-line(yes Marino made them look better than they probably were) and superstar WRs. Even drafting middle to late, he should have been able to acquire the pieces necessary to get to more than one SuperBowl in an era when the AC was so weak.