If in 10-15 years we see a bunch of broke old MMA fighters it will be for the same reason we see a bunch of broke old NFL players, or NBA players, etc. People who make bad decisions with their money. If you get into MMA with the idea of making 10M a fight, you are in the wrong business. MMA is still in it's childhood and while there are millionaires being made, most won't make it to that level, and they shouldn't. The base pay will go up, but fighting two to three times a year on an undercard, going 1-2 or 2-1 in boring decision fights should not make you wealthy.
The long and short of it, you sign the contract and know what you are getting into. Plus we know from countless sources that the UFC pays a lot more money than their contract stipulates. So I don't feel bad for anyone over money woes.