The UFC is such a diverse and unique sport I don't think you can compare it to a league like the NFL,NBA,etc. It will always be the evolution of boxing in my opinion,and will always follow that model. You sell fights and watch it for entertainment and follow a few fighters. Dana white is the perfect fit for the UFC.
The only way I see the UFC becoming a legit sport is if they break that PPV model, which will be impossible to do. Until then it is what it is which is prize fighting. The UFC will continue to grow, however I don't ever seeing it being a powerhouse like a legit sport league. The thing with the UFC its one and out, like you lose its over, you set up the biggest fights there is a drawback.