> The rational action is always to defect.

If that was true, then every bank every day would see runs on their holdings. Obviously that doesn't happen, because most traditional banks have larger institutional customers who understand that the market-to-market value on bonds might dip a bit, but that as long as the bank is able to negotiation loans or raise capital to provide needed liquidity then it's not a big issue. You don't see runs on traditional banks like this, because in the traditional banking sector people pick up the phone and talk to each other instead of taking to twitter and screaming doom and destruction because they read a blog post that they didn't understand but which said something about market-to-market insolvent and sounded scary.