I can pretty much guarantee you that any fan who posted a video of himself/herself setting all of their NFL merchandise on fire is never coming back.
It's even worse than that for the NFL. The absolute worst thing that can happen is for a sizable chunk of their fan base to avoid watching any NFL games for even one weekend. Once these fans realize how much of an idiotic waste of time it has been for them to sit in from of the TV for 3+ hours just to watch 12-15 minutes of football, they're never coming back.
And it’s a one way shift. There are many people who will stop watching over this. But how many who haven’t watched NFL games will start because they sympathize with the players? I would guess that number is vanishingly small.
Parents who stop watching the games will pass it on to their kids as well. They’ll lose a chunk of the next generation.