From Matthew Yglesias, excerpted without comment:
Note that Robert’s debts aren’t some kind of countercyclical stabilization policy. This is an agricultural economy governed by a real business cycle. Nor is he going into debt to finance productive investments. He’s not improving Westeros’ infrastructure. He’s not strengthening the Wall. He’s throwing feasts and tourneys. If he doesn’t have the money he needs today, then he’s not going to have the money he needs tomorrow either. That means a fortiori that he’s not going to have the money to pay Tywin back. Making things worse, winter is coming! The Iron Throne needs to be saving during the fall to deal with the huge negative shock to income that’s coming during the winter.
In effect, Tywin is attempting to execute a debt-for-equity swap since his debts aren’t actually recoverable. But that simply underscores the extent to which the loans to the Iron Throne are, themselves, worthless as financial assets.