I'll start this by admitting that the easiest answer to the above question in the article's headline would be winning a Super Bowl.
Yes, I'm aware that, to many, the only metric for whether a season is successful is whether or not the team they root for wins the big one.
But in all reality, only one team out of 32 can actually hoist the trophy up at the end of the season.
Success looks different to all fan bases.
To a Kansas City Chiefs fan, if the team doesn't win a Super Bowl, the season would probably be considered a failure.