The curious thing is that you can do the same thing with your "socially useful instrument" example. Suppose that instead of selling mortgages, Wall St. had used the same tactics to sell crop futures. They had gone to farmers who promised to provide more food than their land could produce in exchange for cash up front, then resold those contracts at a profit to "investors" and walked away.

What would happen? At first food prices would fall, as in the mortgage crisis loan interest rates fell, because the supply of food on paper has increased. This would cause consumption to increase: Lower price, higher demand. The demand would have to be met from current food stocks because you can't eat securities derivatives, so food reserves would begin to deplete. It would also cause future actual supply to be reduced: Lower price, fewer suppliers. The farmers who can't profit at the artificially lower price would go out of business and stop planting.

Then, next season, the contracts would come due. The farmers who promised more food than they could deliver would default on their obligations. Their inability deliver wouldn't be able to be met from food reserves, which had been depleted when the price was low, nor from other farmers, who declined to plant crops last season when they expected doing so to be unprofitable. Instead of the housing crash there would have been a famine.