All those fighters you mentioned are exceptions, they are UFC champions or at least big draws. They are a huge minority. Even the likes of Nate Diaz are a minority.
The majority of guys fight in very small leagues for a very very small amount of money.
If they want to survive, they definitely need a regular job.
Unless you are some really really good fighter and have at least some good contacts, you will not survive with MMA being your only profession.