Find a name and see what they are doing these days. Sometimes its a big name. Ken shamrock is broke, mayhem is in bad shape, kevin randleman RIP, mark coleman, it's not hard. I don't know who you're a fan of or know about. I can tell you they aren't all living large like bas rutten, royce gracie, or liddel. Guy mezger might have a decent amount of money but he's fucked up really bad from head trauma. It was sad to watch. Chris lytle isn't in the best place for all of his efforts but not the worst either. I had to go through the work digging up these people, you can do the same. I have no incentive to make this shit up. I wish all of them the best, but that's not the norm.
Mark Kerr is another one thats easy to find.
I hear you man... I guess it was a callout on saying "most" fighters end up with brain damage and destitute, when in fact the overwhelming majority, perhaps 99%, end up somewhere on the "normal" spectrum of both health and income.
Looking at the names you mentioned and equating them as the "norm" is not a fair representation. These are clearly the exception and not the rule:
Ken Shamrock has a net worth of 1.5 million.
Mayham Miller (who I run into and speak with on occasion, he trains and lives near me in OC, comes out to lots of ammy shows) is clearly mentally ill, and there is no evidence that it's the result of getting punched. Trust me, he is financially very comfortable.
Randleman - RIP and no disrespect, made MILLIONS between his dozens of fights in the UFC and Pride and clearly abused steroids. The fact that his girlfriend was soliciting donations after his death doesnt mean he was broke, or that he was was responsible with his finances.
Coleman - posterchild steroid abuser and likely alcoholic, by his own admission, blew all his money on hookers and booze and stupid shit. He also made MILLIONS in UFC and Pride, but didnt save enough to pay for health insurance, a 50k surgery, or ensure the future of his children.
Kerr - self admitted alcoholic and drug addict (no disrespect, I am a recovering alc/addict myself). Also made MILLIONS in UFC and Pride but showed very poor money management and responsibility.
Tyson - Career acoholic and obvious irresponsible with finances.
Can you see the pattern? These individuals made poor decisions.