All loans could in theory be paid back, extinguishing all debt. If the loaners are not able or willing to pay back, the loans could be written off as a loss, same result in the aggregate.
Defaulting is not paying back, so that first sentence is incorrect. Due to interest on debit being higher overall than interest on credit, there is always more debit than credit in the system, and debit will always grow faster. If everyone would pay their debts or otherwise default and lose any collateral, the banks would end up with most of the world's property. That by itself shows the banks being parasites, since they never created anything of value themselves, they only moved and manipulated money, yet they'd own almost everything.