Supply and demand in the free market. If someone is willing to pay an asking price, the person asking for that price will sell.
If this doesn't happen through traditional exchanges, it will happen on P2P exchanges.
Discussion
I agree. We will still trade. But at the point when the only exchanges are p2p, why would anyone want dollars?
At some point, insured "exchanges" won't have any bitcoin. And They will serve no purpose.
Exchanges are banks. One day soon, no one will want to buy dollars.