I would say you are right on that because operating costs most of these sports would not compare to a full time league like the UFC.
From what I see, in 30 years the UFC has built a entire league that is number 2 in combat sports fighter pay. Not bad
Along with ad revenues, gates, concessions, ESPN app partnership, PPV revenue....we can go back and forth all day but the main issue with fighters not getting paid more in the UFC comes down to corporate greed & control. A major problem in the world that goes unregulated in a multitude of industries.