I think interest in football is waning because the sport is a lot less fun to watch now that we know those bone-crushing hits actually leave guys debilitated for life. And not just in the long-term; Aaron Hernandez should be a wake-up call for the risks of CTE among active players. A broken leg from a sport is one thing, but widespread (something like 95% of former players show signs of CTE) traumatic brain injury is another.

It's just not right to encourage people to do that, and I think a lot of fans are choosing to watch something else.

Edit: Also, basketball has better economics and a more international fan base, so it's probably safe. Baseball is probably fucked too, at least in the current incarnation of MLB. But the NFL is definitely fucked.