That has nothing to do with TV viewership, which is how the vast majority of NFL fans watch the games. And it's TV viewership that is off 15%.
They just can't figure it out, or they just don't want to figure it ou.
I think the author may have been referencing those changes in ticket prices over the decades as illustrating how the NFL has embraced the elite, and abandoned the middle and lower classes that support professional sports via TV watching more so than any other segments of society by sheer numbers alone.
So for years, I had a tradition of hosting NFL games at my home on the large screen, where the beers are much cheaper and the food options much better! When I ditched cable a few years back, I'd spend Sunday afternoons at Buffalo Wild Wings or some other sports bar, downing pounds of chicken wings and gallons of beer surrounded by big screens. I actually got to like that routine.
But this year, I have not seen a second of NFL football. The protests against the national anthem have definitely been a large part of the reason why but also I'm sick of other facets of NFL football. The suspension of Tom Brady this year for that silly "under inflation of footballs" controversy is another factor. Another factor is the increasing number of thugs, some felons, that are represented on teams these days. It's quickly going the way of NBA basketball, which I stopped watching entirely back in the 1990s.