People think it can't happen, but just look at the history of pro boxing and TV.
For a period in the 50s-60s, pro boxing was the TV sports king.
When the fans get turned off, for whatever reason, or if you intentionally piss down their leg and tell them it's raining, and they leave, you gotta big problem.
I doubt that will even happen. Football teams are owned by billionaires. Every team can fold tomorrow and the owners will still be billionaires. If viewership and support get so low, the billionaire businessmen will sell their sh*t or shut it down, take the loss and write it off on their taxes for the next ten years. The only folks hurt by all of this are the folks that work at the venues, the blue collar folks just trying to make a buck.