They have plenty of options...security, coaching, sports education, school teacher, gym, nutritionist, strength and conditioning coach, television, fight promoter...really not that hard.
It sucks that a lot of them put in so much work but don't seem to reap the same rewards as high level boxers or football players etc, but a lot of the time they're just completely stupid with their money and just can't handle adjusting back to normality.
Really, mid tier guys who fight on main or co main events, have a few sponsors, few bonuses etc should be able to invest their money wisely into property, shares, pay off a mortgage etc and have a pretty fucking good start to their new career, much better than others so I don't always have sympathy for them when they have a big cry about how poor they are post fighting.
Although some guys have a really rough career especially among the old school shooto, pride, pancrase, vale tudo etc type breed of fighters who literally fought for chump change.