There's a buffer between the asset value that the banks are meant to hold and the deposits that they owe back to their customers. And if that buffer gets exceeded, then the bank is technically has negative equity. But it doesn't mean that depositors end up with zero. It means instead of getting 100 cents on the dollar, they get 93 cents on the dollars. If people stopped moving deposits around, then you're right. The banks wouldn't need to borrow money to give the positive their money or go do the work of selling those bonds into the market.