Your line of thinking is mostly flawed, since you may not simply decide to sell your coins for "X" dollars and it be magically executed, depriving other traders from getting any and collapsing the exchange. Most crypto coins like BTC or ETH are essentially "zero sum" games. You may call crypto any scammy name you want, but a Ponzi it is not.
In an exchange you buy coins *from other users* and NOT from the exchange, unless the exchange is a "user" of itself as well. For someone to buy some, someone has to sell.
If suddenly everybody wanted to sell, the price would collapse, since now there are a lot of coins for not so much dollars (from users willing to buy them) but in the end <new price> x <amount of coins> is _exactly_ equal to the amount of dollars available from buyers.
And the exchange, as usual, makes a pretty penny from their share of the transactions.
In that sense, users would lose a lot of money because they bought high and sold very low (close to, but not equal to zero), but would never get nothing from their coins, barring exit scams, "hackers" stealing the wallets, etc. In the worst case of the exchange not having liquity for that coin, nothing blocks transfer to another exchange where there would be a little value left.
OK, I will concede the case where the coin collapses so hard that it has _zero_ liquidity, then you will lose exactly the value you "invested" and not one cent more.
However, if you start "investing" in derivatives, futures and other complex "financial instruments" involving crypto, then you may lose your pants. For example, if you bet that the value would go up and borrowed a lot of money to buy crypto now, and the value crashed, you'd be in deep trouble.