Some economists argue that the market can tell us what the probabilities are. We can show how this is supposed to work by turning our current example around. The probability we want is the number p such that 9756 times p equals 8780. Doing this in real life could be more complicated. In reality, there are more possible outcomes than just paying in full or paying nothing.
Payment can be partial, or it can be deferred for a while.
Economists argue that the market price, set by the interplay of supply and demand, reflects the probabilities of all possible outcomes. The more basic problem with inferring probabilities from market prices is that market reflects what people think. What people think can be wrong, and it can change rapidly. In the summer of 1998, after Russia temporarily stopped paying off its bonds, market prices fell sharply for Brazil's bills and bonds. The connections between the Russian and Brazilian economies were too thin for there to have been any direct causation.