Pundits will point out that, as recently as 2010, the Broncos were 4-12. As recently as 2011, they employed Tim Tebow. They'll mention that Seattle had back-to-back 7-9 seasons in 2010 and 2011. Both teams made the playoffs last season, so it's not a classic "worst to first" story for either of them, but it's quite a change in fortunes from just two seasons ago.
But, I don't think that captures the whole truth. I think the NFL has a great deal more parity than the NBA -- another league with a salary cap and free agency structure -- where the Heat, Celtics, Lakers, and Spurs have won almost every championship since 1998. The NHL has been through so many transitions since the turn of the century that it's really tough to try to compare apples to apples -- there are just too many variables.
That leaves Major League Baseball, home of no salary cap and the haves/have nots club (Yankees, Cardinals, Red Sox, Dodgers, possibly Giants and Angels). Since really only the Yankees, Red Sox, and Cardinals have a shot to win it every year, baseball should take a long look at what the NFL is doing and try to parity themselves up... right? Given, the title of this post, I think you can safely assume the answer to that is wrong.
First of all, there's the stat that everyone seems to be mentioning, but glossing over: Peyton Manning, Tom Brady, or Ben Roethlisberger have started at quarterback for the AFC representative in 11 of the last 13 Super Bowls. That basically means that the other 13 teams in the conference had no chance if they didn't have themselves a franchise quarterback. Add in the fact that the hated Ravens were the AFC representative in 2000 and 2012, and Rich Gannon's Oakland Raiders are the only real outlier.
Since 2000, nine different teams have won the Super Bowl, with four teams winning multiple titles (Patroits, Steelers, Giants, hated Ravens).
Since 2000, nine different teams have won the World Series, with four teams winning multiple titles (Cardinals, Yankees, Giants, and Red Sox).
Seems pretty balanced, even though baseball doesn't have all the parity enforcing rules in place that football does. Add to the fact that the playoff field for baseball is 8 teams a year (or had been until last season), while football admits 12 teams to the postseason, and it's pretty remarkable that the scales are that balanced. Then, think about the fact that NFL playoff games are single elimination and MLB playoffs are five and seven game series. In a long enough series, the team that can buy the most talent should be able to outlast a less expensive, less talented team.
The balance is almost as good when you go back further, even though the Yankees won four titles between 1996 and 2000.
Since the baseball strike that canceled the 1994 World Series, ten different teams have won World Series, with five teams winning multiple titles (Marlins, Cardinals, Yankees, Giants, and Red Sox).
Since free agency started in 1994*, 12 different teams have won the Super Bowl, with six teams winning multiple titles (Broncos, Packers, Patriots, Steelers, Giants, hated Ravens).
*I know free agency officially started in 1993, but they really didn't work out the kinks until '94 or '95.
If you go all the way back to the beginning of the Super Bowl era, it gets even more interesting, even though you also had clear baseball dynasties in the 60s (Yankees again) and 70s (Reds, A's), though no team emerged in baseball the way the 49ers did in football (ten different teams won the World Series in the 80s, a different team in each year).
Since 1966, 18 teams have combined to win all 48 Super Bowls. There are 14 teams that have not won the Super Bowl, and 4 teams have not been to the Big Game.
In baseball, 20 teams have combined to win 48 World Series, only 7 teams have not won (half the NFL number), and three teams have not been to the World Series. Hey, even the Pirates logged a couple of wins.
So, the parity in both sports is more like the parity in Men's NCAA basketball: There a lot of different teams that qualify for the postseason, but only a handful of teams have a real shot at the title. Every now and again, there's an upset, but it's usually Duke, Kansas, UNC, Louisville, Kentucky, UConn, and Arizona (maybe Syracuse in recent years) that comes away with the big prize.
At this point in the NFL, you can't win it all if you don't have a franchise quarterback (Joe Flacco is totally elite, you guys). Before the rule change in 2003 that opened up the passing game, back to about 1990, you needed a great defense to win championships. From the last big rule change in 1978 through 1990, you needed a franchise quarterback and talented receivers. Prior to that, defense won championships for the most part.
Dallas won with their Doomsday Defense in the late 60s and early 70s, then shifted to offense with Roger Staubach and Tony Dorsett when the rules opened up. The Steelers went through the same progression. You could make a strong argument that Terry Bradshaw was the third best quarterback on the team that won Super Bowl 9. He wasn't that much improved by Super Bowl 10, but he was starting to get on solid footing, then they rode his right arm to titles in Super Bowls 13 and 14. Don Shula went to four Super Bowls (6, 7, 8, 17) and won two with defense, then re-booted and went to the Big Game with Dan Marino and a record-setting offense in Super Bowl 19.
The Cowboys got back to defense and a solid running game, for the most part, in the 90s to give themselves another title run. The Patriots started out with a focus on defense, then changed as the rules and game changed in order to sustain success.
But, the common thread across all eras seems to be quarterback play. John Elway played in a Super Bowl in 1987 and 1997 and 1998. Joe Montana and Steve Young were able to give the 49ers a nice, long run of success. Tom Brady has been to five Super Bowls (so has Elway). So, parity hasn't been able to level the playing field if a team happens to draft a once-in-a-generation guy. Think about the stat I referenced at the beginning: Three quarterbacks have dominated the AFC for the past 13 years, amid changes in playing styles and rules and schemes.
In baseball, a lot of it comes down to luck as well. The richer teams have just been more consistently successful because they have the money to hedge more bets -- and safer bets on more expensive players -- than other teams.
In football, the most consistently effective teams are able to set themselves up to take advantage of any lucky breaks that they get. The Steelers struck while the iron was hot in the 70s -- they had the best run of scouting and luck in history from 1970-1974 -- but struggled in the 80s because luck wasn't on their side. They re-tooled in the 90s and had some success because of their defense, but didn't really get another lucky hit until Roethlisberger in 2004. They were set up well at that point to succeed, they just needed the final piece.
Basically, success is based too much in luck, right place/right time, and positioning yourself well to take full advantage.
To say that one sport has a model that can tame that beast is silly. That's why NFL parity is a myth.