These guys are "independent contractors" by design, so the UFC doesn't have to contribute taxes or afford them the typical benefits of an employee, but want to control everything about their career.
This needs to cut both ways - these guys don't work for the UFC, and should be advocating for whatever fights make them the most money. You want to have complete control over them, then pay them a remotely reasonable split of the revenue from their fights.
For 15% of revenue going to fighters, these guys should all absolutely be calling for the fights they want, not kiss the ring of billionaire class promoters keeping 85% of the cash coming in.