I don't have any numbers to prove this but I believe the decline of american football has already begun and will continue although it will never go away completely. I believe it will evolve and become more of a fast-paced, high scoring, low contact game. Some variation of 7-on-7 may become the norm in 30-40 years especially in high school. I base this on what appears to be a decline in the interest at a young age which is probably a combination of expense and injuries. More and more children are playing soccer so I think you will see that continue to increase in popularity at the expense of football. IMHO this is the main reason why southern schools are dominating college football as interest seems to have faded in other places at a faster rate than in the south but I expect that to happen here as well over time. If my theory is true then football will have to evolve to survive.