It be erosion from the bottom up
From the bottom up agewise and from the top down socially. My dad was a college football coach and then athletic director. One of the things he used to say is that if you look at "tough" sports, they tend to be dominated by people with fewer options. Someone mentioned boxing, and he used to like to argue that the reason so many great boxers were Irish and Italian and that's not true anymore is not because Irish and Italians don't have the genetics to compete against Hispanics and African-Americans in boxing, but they don't have the need to get punched in the face to make a living.
He made a similar argument about quarterbacks versus linebackers.
I think what you'll see is that wealthier families, who are saving for college for their kids and are counting on sports to keep their kid rounded and education to keep their kid employed, will start pushing their kids away from football. I see it already in terms of soccer (aka "real football" for the non-US audience following along) becoming the preferred sport of the upper middle and wealthy classes on the coasts. And despite some fear-mongering, it is almost certainly a lot safer
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https://sports.vice.com/en_us/article/qkyyv3/the-study-on-cte-in-soccer-is-not-what-you-think-it-isPlus, I see an evolution of sports perception anyway. When I was a high-school rock climber, this was an absolute geek sport. Nobody could name a climber they didn't know personally. It was not considered a sport at all and those who climbed were not considered athletes in any way. It was just some weird sort of camping for asocial nerds (maybe I'm exaggerating a bit, but I barely remember anyone even asking me about climbing or being in the least curious when I was a teenager).
Now I have a co-worker who told me she wants to have Tommy Caldwell's babies (which is funny, because Tommy is a friend and as humble, relaxed and geeky guy as you could imagine) and Dean Potter had all kinds of groupies (though also a neighbor and as nice a guy as you could meet). I think there are a lot of activities that have been around a long time as "nerd fringe" that are now "cool" and semi-mainstream.
More and more, I think the "cool" kids in a high school are the ones *not* playing football. May be different in the Midwest and Texas, but it's already true in a lot of the country.