I think it is happening to the NBA. Despite some very good playoffs, I think fans are tired of the constant thuggery of many of the players, and the relentless favoring of specific "superstars." Ever since Michael Jordan left the Bulls, basketball ratings have dropped dramatically. I've never been a baseball fan, but there seems to have been a little resurgence there, of late. Barry Bonds and steroids could kill it. The NFL is riding a wave, and while it could collapse, I don't think it will. College football is the closest competitor. Let's face it, which game was more exciting, the Rose Bowl or the Super Bowl? College ball and the NFL have kind of a "non-competition" agreement, and the NFL doesn't play on Saturday until college football season is over. I've also noticed that towns with great college teams have little interest in having an NFL franchise (I've always believed that people in LA follow USC and UCLA, and then kind of scan different NFL teams). If college football started scheduling games on Sunday, which ain't gonna happen, they could hurt NFL ratings. Nothing else can come close.
Football is made for television, far more than baseball, basketball or hockey. If ESPN dumps hockey, where are they going to go? Home Shopping Network?