If civilization collapses, 7 billion people will die, at least.
The economy contracting 90% is a possibility, and if your income goes to zero, you don't want to lose what you have left because of a loan you didn't think you'd actually have to pay off.
If the economy contracted 90% you wouldn't have to pay any loans because the loan collectors would be dead. This is sort of like "if you owe the bank a billion dollars the bank has a problem".
Only the unsuccessful ones.
collapse != instant mad max.
Legal claims will very likely still exist. And if you owe money to somebody with more resources or power than you then you're gonna have a bad time.
Look at the USSR in the early 90s for a great example of what that might look like.
Why would you pay off your debt if you thought civilization was going to collapse?