Will a bank ever reduce the amount of your mortgage if their own appraiser was full of shit?

I bought a house at the top of the bubble for a sentimental reason without negotiating down by the standard (at that time) 20 per cent below the asking price. The bank’s appraiser went along with the asking price – transparently a way to help the bank justify loaning out as much as possible. The house was never worth that much. And THEN the bubble burst. Like many people, I now owe much more than the house is worth. Why should the bank profit?
Yes, this is a question shared by many.