Would there be enough money in the exchanges? Would bitcoin fail?
If EVERYBODY wanted to dump all their BTC, the price would go to ~$0/
BTC... So yes, in this case there would be enough FIAT on the exchanges.
It's all supply and demand. If the supply is huge and the demand is 0, the price goes trough the floor.
It's not clear to time if exchanges would have enough liquidity to meet the demands of such an insane sell off. I mean it's not even realistic. We are looking at a $94,323,939,141 marketcap as of right now. Are you saying bitcoin exchanges would have that liquidity ready? I don't think so, they would probably be facing bankruptcy. If bitcoin goes to $0, their whole business is a failure because they bet on something that failed.
Luckily for us there is no need to worry about that, that will never happen.