We hear it all the time--the NFL is a league of parity. There are no juggernauts. Any team can come out of nowhere and end up in the Super Bowl. Indeed, teams like the '99 Rams, '01 Patriots, and '03 Panthers make it seem as if a bad team can suddenly make a run to championship contention overnight.
There's a great deal of myth here. Yes, in the NFL, any team has a good opportunity to compete regardless of location. Yes, free agency makes fast rebuilding projects possible.
But this isn't a parity-driven league--good teams tend to stay good for a number of years. Let's look at the last 7 Super Bowls to see.
1999 St. Louis d. Tennessee
The Rams did come out of nowhere, but it was in fact the beginning of a pretty successful run. From '99-'04, the Rams were in the playoffs 5 of 6 years, and went to another Super Bowl. Including '99, the Titans went to the playoffs 4 of 5 years through '03.
2000 Ravens d. Giants
Here's a case of potential flash-in-the-pan teams. The Ravens hadn't been to the playoffs since moving to Baltimore, and would appear in the playoffs again in '01 before rebuilding. The Giants had previously made the playoffs in '97, and would make it again in '02 before tearing it apart and rebuilding.
2001 Patriots d. Rams
The Rams are already spoken of. The Patriots seemed to come out of nowhere, but this was really just the beginning of a dynastic run. They've followed the '01 Super Bowl season with four straight winning seasons, three playoff appearances, and two more Super Bowl wins.
2002 Buccaneers d. Raiders
Before winning this Super Bowl, the Bucs were in the '97, '99, '00, and '01 playoffs, including the '99 NFC title game. Before losing this Super Bowl, the Raiders were in the playoffs in '00 and '01, including the '01 AFC title game. This matchup culminated the runs of two teams that were in contention for a while.
2003 Patriots d. Panthers
The Patriots are in the middle of their run here. The Panthers recovered very quickly from a 1-15 season in '01, but since then, it appears that John Fox has built a winning football team. The Panthers were back in the '05 NFC title game, and don't appear to be going anywhere.
2004 Patriots d. Eagles
The Patriots again. Before 2004, the Eagles were in the playoffs 4 straight years and in the NFC championship game three straight years.
2005 Steelers d. Seahawks
The Steelers had been to the playoffs in '01, '02, and '04, including hosting the AFC championship game in '01 and '04. For the Seahawks, 'o5 was their third straight playoff appearance.
This list of Super Bowl teams isn't a great deal different than any other era. Teams that are good stay good for a few years, breaking through to the Super Bowl one or more times during their runs of contention. The NFL from '99-'01 actually compares pretty well to the NFL from '80-'82. At that time, the Steelers' dynasty had pretty much wrapped up. The Cowboys were still contending under Danny White (they did lose the NFC title game all three of those years), but their run of championships was over. The Vikings were done losing Super Bowls for a while. And during this time, the Raiders became the first wild card team to win a Super Bowl, defeating the Eagles (who were in the middle of four straight playoff appearances). The Raiders won with a different coach and QB than they won with in '76--one could argue it was a different team. In '81, the 49ers made it to their first Super Bowl and won. They hadn't made the playoffs since '72--you could argue that they came out of nowhere to win the Super Bowl. Of course, we found out they were to be the dominant team of the '80s (actually, from '81 to '97 they were in the NFC title game 10 times). Then in '82 the R*dsk*ns, who hadn't been to the playoffs since '76, won the Super Bowl in a strike shortened season.
If you look at the period from '80-'82, you'd think you were watching a league of parity, where teams can come out of nowhere to win a Super Bowl. In fact, the Raiders were in the middle of winning their 2nd of 3 Super Bowls in 8 years, the 49ers won their 1st of 5 Super Bowls in the next 14 years (4 in 9), and the Redskins would be playing in 4 Super Bowls, winning 3, in a 10 year period.
That was a period of transition, just as '99-'01 was a year of transition. It was a period between the Dallas-San Francisco-Green Bay-Denver success of the '90s and the Patriots success of the 'oos.
It's still a league of juggernauts. Don't let anybody tell you differently. It's just that the NFL has a system in place to maintain competitive balance regardless of location, and the free agency system makes it easier for rebuilding teams to rebuild within a matter of a year or two. That's notable--but it's not led to a drastic change from the history of the NFL.