The Bears switched to a traditional 4-3 defense immediately after their Super Bowl win in 1985-86, primarily because Buddy Ryan left the team to become the head coach of the Philadelphia Eagles. Defensively, the Eagles were never as dominant as the Bears had been -- mainly because they simply didn't have the same level of talent as Chicago.
Despite their perfect record, I wouldn't even include the 1972 Dolphins among the top ten teams of all time. They played a very weak schedule that year, and were so unimpressive in running the table that they were actually the underdog in the Super Bowl against Washington.
I well remember the Skins. If they had had a real QB, they would have been nearly unstoppable, but Billy Kilmer was slightly better than my uncle (and he's dead).