You are correct, however, those teams you mention didn't just have a good defense, they were a dominating defense, and more specifically a scoring defense. Those type of defenses happen rarely, and it's near impossible to keep that up, offensive coordinators always find a way to neutralize that defense. That's why the 1985 Bears won only one SB, and why the Broncos didn't even make the playoffs in 2016 after the defense "carried a half-dead Payton Manning" to a SB win. Now sure, Manning retired after the SB win, but if the defense continued their dominating ways, even with another mediocre quarterback they should have at least made the playoffs.