The decline in the quality of the NFL product lies largely in the rules changes giving an advantage to the passing offenses. The most valuable commodity in the NFL is a franchise QB and there are few true franchise QB's. As of last year, Tom Brady, Peyton Manning, Ben Roethlisberger, Aaron Rodgers, and maybe Eli fit that bill. Guys like Brees, Rivers, Newton, Wilson, and Ryan are really good but not great.
With so much emphasis put on the passing game and only 4-5 stars at 32 positions leads to a large group of slightly above average to slightly below average teams. There have been a few break through seasons by teams, but no SB loser has made it back the next year since the Bills in 1993. And during the 2000's decade only the 2006 Seahawks and 2009 Cardinals made the playoffs the next season 8 other teams failed including the 2007 Bears.
Now the NFL will never do it, but if the rules were such that the running game became the staple of a good team again, the overall quality of the product would be improved. There are more really good, and less spread of high to low quality RB's than QB's. With more running there would be less crap calls in the biggest crap call category(PI, catch/no catch). One key injury would not doom a teams season.