recently the majority of people see btc and use it as an investment opportunity. If there won`t be liquidity in market than the demand will fall sharply and even if someone would like to dump their coins they won`t be able to find any buyer which would make the coins without any value.
Even if there is no connection between all exchanges, they already have the bucks and that is what makes them an exchange anyway. The thing with most exchanges is that they trade too, and they make a lot of money from these fluctuations, so at the end, there should be enough to go around.
Asides that too, I really do not trust any exchange if ever a scenario like that happens, they may just simply close down or stop withdrawals, but we cannot see a situation like this anyway.