The banks can lose money if the property prices fall is sustained. When the market falls and the mortgagee cannot make the monthly loan payments then after some time the banks would have to take possession of the property and eventually auction it off. In a falling market the bank may not recover the amount loaned because during such times the initial reserve price may not be met and they would have to just sell it to the highest bidder.
http://budgeting.thenest.com/pay-after-repossession-20126.html
Once you sign a loan contract, you are obligated to pay the entire amount owed on your car. Repossession doesn't negate that contract. If you can't afford the car and the bank repossesses it, you still owe the balance of the loan. Typically, the bank sells the car at auction if you can't afford to pay the past-due balance. You are obligated to pay the loan company the difference between what you owe and what the car sells for. For example, if you owe $10,000 on your car but it sells at auction for $6,000, you still owe the loan company $4,000. The lender may let you make payments, but it's more likely the company will sue you for the amount in full.
I'm not even incl. interest that they've earned in the past, nor the interest on the o/s sum after they repo. How the fuck is the bank going to make a loss, unless everyone goes bankrupt?