There are a lot of exchanges. But YOU (or any person in particular) trying to cash out at any point in time will need to move your coins to that exchange (and create an account, etc), and then try to sell. There aren't a lot of buyers when a bubble is popping. And not much potential for arbitrage when everything is going down. And with Bitcoin's slow confirmation rates, I think it would be worse than a typical stock market. No, if the bubble pops, it's going to be bad for everyone involved.