[waqf]

The optimal way to play any "reputation" system is to build up an exemplary rating until you've unlocked all the "trusted-people-only" privileges, then suddenly exploit them all at once and disappear. In the case of credit ratings, this means being an exemplary debtor until one day you're trusted with a huge loan and you default.

This clearly still happens. It's a fundamental fact about how debts work and it's a central element in many confidence tricks (for example pyramid schemes). So credit ratings only benefit the lenders if the amount they save on loans to suboptimal players who default at random exceeds the amount they lose to organized people who exploit the system.