They bought the UFC for like a million dollars. Zuffa is an LLC, which means that the Fertitta's personal assets could not be taken by the debts of the company. But if the UFC had made no money they would still have lost all of their initial investment.
And even if they don't risk their own personal wealth they are still the ones procuring investors and negotiating contracts with sponsors.
The revenue is generated by people who pay to watch fights, not necessarily certain fighters. There are hundreds of talented fighters outside the UFC who could take the place of current UFC fighters.
Why do you think franchises like Star Wars hire unknown actors? Because they can do the same job for much cheaper. Sure, they may not be able to act as well as a big, well known star, but people pay to watch Star Wars, not to watch a big star. In other words people will watch even if you don't have the best.
I know this argument only holds up to a certain point. You are right that fighters like Mighty Mouse bring legitimacy that makes the UFC sustainable in the long run. But the UFC has proven that it DOESN'T need ALL the best fighters in the world in order to get viewers. They never had the best heavyweight in the world, Fedor. They don't currently have two top fighters in Rory Macdonald or Benson Henderson. It is a balancing act that the UFC knows how to play between having top fighters and having money making fighters.