There is no guarantee people in this situation must fail. They might get a raise, or a promotion, or a surprise inheritance, or win the lottery, or who knows.

Now, obviously you don't want to get into this situation in the first place, if you can help it. But if a person is, then they are not simply doomed as you're saying.

We should not excuse exploitation of people living paycheck-to-paycheck simply on the basis that they're bound to get fucked over eventually so it doesn't matter when or how it happens.