The NFL has a famous phrase to capture the idea that the league leads the way when it comes to parity and anyone being able to win–“any given Sunday.” Everyone is supposed to go 8-8, and you hear about the worst-to-first stories.
We’ve also seen teams have a hard time winning multiple championships recently. Seattle came within a goal line play of repeating. New England twice lost to the Giants in crazy fashion. The Giants are the only team to win two Super Bowls in the last decade, and they did so after regular seasons with 10 and 9 wins.
By pretty much any other measure, though, the NFL has been far more of an oligarchy than the other professional leagues. Fewer teams advance deep in the playoffs, and the teams that have been good in the past tend to remain good at a higher rate than other professional sports. The rags-to-riches tales have been fewer and far between in the NFL compared to other sports over the last 15 years. The rich have stayed fairly wealthy. The Patriots had the best record in the NFL from 2001-2005. The Lions had the worst. From 2006-2010, the Patriots had the best record; the Lions the worst. From 2011-2015, the Patriots again are #1 (the Lions have climbed to mediocrity more recently, but still no playoff wins).