I think it can happen naturally if they can fix players resting. The sport of football itself is falling apart. People will stop letting their kids play football. I think it will suffer a similar fate to boxing. The NBA dominates the internet it seems to me in terms of discussion, youtube, social media, etc. and NBA players are far bigger stars than NFL players with 1/10th the off court problems. In the era of social media this is a big leg up for the NBA. Look at the amount of views and hype and how everyone talks about high school ballislife mixtapes, that shyt's crazy.
The NBA is more progressive and I think has superior longevity. Who knows how long it'll take, but I see the NFL imploding eventually.
Basically, the NBA will continue to grow at a rapid rate and the NFL will eventually kill itself I think. It won't be a normal takeover.