Over here, a lot of the people who make it in the top sport and earn stupid money even with taxes end up totally ruined within years of retirement. They earn so much during their career that they have a whole lot of parasites surrounding them 24/24 sucking their money, have zero money management skills, and little to no skillset to sell afterwards.
A lot also get depression from the brutal changes that happen at the end of their career. And I'm talking about guys that are in that sport since they're five and would have had a lot of time to learn how to be a functioning adults.
In the end, I can imagine that fighters that often begin MMA at 20+, even if they got filthy rich, could really easily waste all that money in a few years