One thing that has gone unrecognized is that thee rule changes instituted to increase after the 1973 season hurt the Dolphins a lot. The 72 and 73 defenses lead the league in scoring defense allowing only 171 and 150 points a game. They were great defenses but oddly, did not have great corners in Tim Foley and Curtis Johnson. There was no Herb Adderly's or Mel Blount's on the squad. Neither Foley or Johnson ever made the Pro Bowl as a corner.
The Super Bowl Champion teams compensated for these corners by running mostly zone. At that time you could hit a receiver all over the field until the ball was thrown. The three linebackers, Doug Swift, Mike Kolen and Bunoconti were small but very fast. The five-yard chuck rule was not implemented until 1974. So the Dolphins linebackers would make deep drops and the zone turned into a gauntlet.
I remember watching a playoff game between Miami play Cincinnati, with Bill Walsh as the OC and Ken Anderson as the QB, where the Dolphins held Anderson to 113 yards passing. The Bengals All-Pro deep receiver, Isacc Curtis, was held to one catch for nine yards. I remember the color guy, (probably Al DeRogatis), harping on the fact that Curtis was getting hit all over the field.
After the rules were changed the Dolphins defenses were generally good but as dominant as the Super Bowl teams. They were much easier to throw on.