This is one of the most important concepts in economics that is tailored to accommodate markets based on fiat currency ... assumed values.
Since paper money is never anchored to anything substantive in the real world, a currency can be destroyed simply by market players deciding the effective value of their holdings (represented by paper currency) has no further value to them.
The entire ponzi scheme depends on a certain assured percentage continuing to believe in the fairy tale of fiat currency and paying mortgages on homes that will never be worth what the bank based the mortgage valuation on. By simply walking away from their property in large numbers, the entire population can effectively deflate the currency by an astronomical sum in very short order. See the blog below on the Zimbabwe dollar.
Fiat currencies work off faith. Currencies denominated in metal backing do not need faith for them to retain their value. If enough people stop believing the lie of paper money, the U.S. economy will turn from a freefall into a death plunge in a matter of weeks. If the bank says, "your home is worth X dollars to you, you must continue playing this game if you ever want to win," and the mortgage owner says "my house is worth zero to me, I reject your implied obligation, go ahead and take it away from me because it's nothing but a burden," then the entire system will implode into itself inside a very short time frame.
All remaining value in the country hinges entirely on a fleet of willing suckers who feel compelled to keep playing the game of musical chairs. There is no fallback position for banks which have their net worth stocked in this dream if their mortage slaves start to abandon their homes.
At the end of the day, the real tangible value of many of these homes is less than $25,000 worth of raw materials that have no resale value as scrap. Everything else is just implied or subjective worth in terms of subjective paper money.