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.
Before it reaches to zero many people will try to convert it to fiat and not sure how exchanges can manage that part because currently, bitcoin prices are very high.