They obviously need to keep dollars on hand in order to cash everyone out. But what if Bitcoin goes up in price? If there is a run on the exchange later they may have to sell their bitcoins on the spot market to cash people out.
Banks reinvest money held into loans and other speculative investments. But what do you do when as an excuange / market maker you stand ready to buy and sell at any time?
Here is my concern: while it was just two guys meeting up to exchange $ for bitcoin, the $ kept circulating. But these exchanges may be locking up more and more USD just so they have enough to cash people out on demand. As a result, there is a real transfer of wealth to bitcoin and USD is getting frozen in exchanges.
If this continues for 5-10 years more, wouldn't bitcoin become millions of dollars and bitcoin holders would be able to buy up anything vs dollar holders? One bitcoin holder could buy half of NYC already, and who can stop them?