If everybody suddenly sold all their USD, EUR, or other currency, that currency's value relative to other currencies would also crash. That's not unique to Bitcoin.
People cash out their currency to buy goods and services, that's the whole point. You accept currency knowing you can spend it later. It's useless in and of itself. In order for them to spend it later, somebody has to be their "exit liquidity" and trade a good or service they have for that currency. You can call that a ponzi scheme if you want, or you can just call it currency, because that's how currency works.