As you're finding out, student loans can not be discharged through bankruptcy. But it is very hard for creditors to get at your income from Social Security. Actually, I think it's close to impossible. (The IRS is one of the few entities that can, if you owe taxes.) Navient is figuring you may someday go back to work. Or you could come into income or assets some other way. You might win the lottery or inherit money. If you're still relatively young, they are not going to assume they won't ever get anything out of you. So the loans stay hanging over your head, just getting bigger with interest accumulating. It's tough to have that looming in the background indefinitely. Get what professional advice you can get, as suggested above. There might be a way to someday get the loans discharged.

Simple default is sometimes a viable way to deal with debt you can't make good on. My boyfriend got sick and defaulted on a large amount of consumer debt. He never did a bankruptcy, but just defaulted. He has since rebuilt his credit to where he has an above average credit score and has access to plenty of credit, some at very reasonable interest rates. The main thing is to let creditors know your only income is something they can't touch.