NFL television viewership was down 9.7% in 2017. Viewership of college football also was down. NFL attendance held pretty steady. The attendance numbers were down about 1%, 2017 vs. 2016. However, they were about even with the 2012/2013 timeframe.
I use to watch most every televised NFL game. This past season, I probably didn't watch 3 games from start to finish. Each year, it seems the game gets a little more stale. And, instead of the Sunday games and Monday night games, we now have Thursday night and Saturday night schedules. They have just worn out their welcome.
Toss into the mix the surge in membership/viewership of the streaming services. Netflix, etc. has grown by leaps and bounds. People are binging GOT, Scandal, etc. instead of watching football.
Certainly, the political issue that has been waved as some sort of 'banner of patriotism' has probably caused a number of previous viewers to seek out other entertainment venues. To blame the entire loss of viewership... often overstated by certain politicians and media sources... on the political issue is hogwash. Other contributory factors that have affected viewership numbers need considered in the big picture.