People taking out a student loan voluntarily took on that debt. Pretty much nobody is voluntarily choosing to get cancer. (Yes, some people perform behavior that increases their chances, but rarely is it a 100% chance)
So if someone chose to do something knowing the negative consequences that would happen (repayment + interest), and then the rules change on them and everyone else in the middle of the contract, can you imagine someone being at least a little unhappy about that?
An analogy that I think fits better:
A teacher says there's an exam next Friday on chapters 1 - 5 from a textbook. You study, do your work, skip spending time with friends and family on the weekend in order to study for the exam.
But then the exam rolls around the teacher says, "Nah, you know what? We're only going to test chapters 4 - 5."
You might think, "But I could have seen my friends instead! I could have spent my limited amount of time in different ways over the past week. The guy who went out and partied all weekend got lucky and will get the same grade as me, ugh." I can certainly understand why someone who studied may feel unhappy about that, or think it's unfair.