You see I think the issue really isnt just that "fighters are underpaid" some of them are but also I think theres a "you get what you pay for" situation in terms of MMA talent.
The fighters earning very little from the UFC honestly a lot of them simply arent that good, maybe there not worth much more than that. The problem is more than because the UFC intro pay is so low your not getting as many good fighters moving to MMA as you might were they more like the 80/20 spilt.
I think its that early pay thats often key, you'll have something in say kickboxing, BJJ, Judo, etc look at MMA and wonder "is it worth the risk switching for this? I could earn decent money 5 years down the line if I'm sucessful but why switch for pennies now?".