Well they wouldn’t so they’d end up having to setup a payment plan with the IRS but that would still be better than the debt load. I’d rather owe 20k to the IRS than 100K in fed loans. At the end of the day it’s all money owed to Uncle Sam.
Personally though, I wouldn’t bank on IBR, I’d much rather consolidate my loans privately at a better rate and pay them off as quick as possible rather than pay a high interest rate to the fed in hopes that they forgive it one day. I understand that’s not an option for everyone though and some value the risk/reward differently.