I disagree. Marv was a great Mentor, but not much of a coach. Not only was he outcoached in four super bowls, but he was totally unwilling to lay down the law with Kelly and his partying. In each of Parcells superbowls, he implemented a restricted curfew before the game for all his players, as have most of the other winning coaches. Not Marv, he left it alone. Marv was great at inspiring his players, especially after the losses, but I question whether that is really the goal of a coach, or not.
Marv had great players that Bill Polian provided. Marv hired old Swizzle Stick as his DC, which shocked everyone. His own players felt he was incompetent as a DC, but they played hard anyway. Marv did a great job keeping the team focused, but I don't see that qualifies as "being a Great Coach". Is the purpose of being a coach to win?