1. Kirk Cousins aging overnight
If Kirk Cousins doesn't fall off a cliff in the middle of the season the Falcons are beating both the Chargers and Saints. Forget season-ending losses to the Commanders and Panthers the Falcons lock up the division before they get there. The Bucs were only able to win the division based on Atlanta's epic collapse.
There are a myriad of reasons for Atlanta's season going wrong but the biggest remains Kirk Cousins. The quarterback was worse than Mariota or Desmond Ridder by the end of the season. Unable to get any power on his throws and consistently giving the ball away because of it. Even with Cousins playing at an abysmal level the Falcons were still consistently in games.
If the version of Kirk Cousins we saw through Atlanta's first nine games sticks around the Falcons are a playoff team. The awful defense, Raheem Morris, and poor situational awareness won't matter. The Falcons had themselves in a position to simply win the layup games and win the NFC South hosting a playoff game in Atlanta. Kirk leading the league in interceptions and losing his fastball without warning is the biggest factor in Atlanta's late-season downfall.