The NFL is not going anywhere despite the mass media effort to make it happen. What will likely happen though is that the game we are watching today will likely be considerably different 15-20 years from now.
The NFL is simply too popular among all sports fans. With most fans, MLB is only interesting the first and last months of the season. The NBA only has 3-4 teams every year that matter. MLS and the NHL only matter to small segments of sports fans.
The only real threat to the NFL comes from NCAA Football and Basketball, and those are facing their own problems as they transition from amateur sports to paid athletes which is a stepping stone to unionizing, CBA and agents.
Who knows .. maybe in 15-20 years, football players will use augmented reality (aka: VR) to play and injuries will be a thing of the past