I think you missed one. Last time I checked the playoffs were part of the season:
NFC Championship L 3-14 vs Carolina (a team they beat earlier in the season)
That was a big letdown (some may say "choke")
your definition of late-season letdown is spurious. did a team that went 10-6, won three playoff games, and lost in the superbowl have a "late-season letdown"? I guess your answer would depend on whether or not you like that team.
the vikings, saints, and packers had "late season let downs" - all lost games that they needed to win. the seahawks had a "late season let down". neither team still playing had a late-season let down, and a loss on sunday doesn't constitute one either - at least not in the eyes of anyone reasonable. hth.