He didn't have average rosters. The years you keep bringing up where he made the playoffs he had a HOF RB who is the best of his era by a country mile, a top 5 WR, a top 10 line, a QB playing like a top 10 QB and a defense with pro bowlers or All Pros at nearly every level.
The closest he had to an "average" roster was the last two years which got him fired. He relied on running Derrick Henry into the dirt and couldn't win a thing the minute AJ Brown walked out the door.
He doesn't elevate his teams. One year with a loaded team he beat a Patriots team that was limping into the playoffs and a Ravens team that has since showed they don't show up in the playoffs and then got downed by real contender. Every other year after besides that he failed to win a playoff game.
We know what he is. He's a guy that coaches to the talent level of his team. If you give him one of the best rosters in the NFL, he'll make the playoffs, but unless he catches two pretenders off guard, he's just going to get beat by a good team with real aspirations. And if he doesn't have that, he'll be mediocre.