@Richard Andreassen
The problem with modifying supply based on an exchange rate is:
Which exchange rate to use? For example, when Bitcoin was created, there were no exchanges around.
It would introduce a lot of uncertainty in the design.
People could just use whatever exchange rate they wanted by putting a url to a service that provided an exchange rate into their client. Then the network would have to agree by majority rule or something.
I think there are bigger challenges to my suggestion than this.