The NFL doesn’t have anywhere NEAR a monopoly on pro-football. There’s the arena league, there’s the UFL, there’s dozens of semi-pro leagues. The NFL is the only one where players can make millions, but if they’re willing to make normal people money there’s plenty of leagues out there.
There’s nothing close to the NFL in terms of what 99.9% of consumers understand to be Pro Football. That’s like saying the NBA has competition from some local basketball league in the Tri State area.