Island's economy would not be an isolated system.
Well neither is Australia's.
But you don't see the price of a Big Mac in Australia changing every day! The price of a Big Mac, in any country, in bitcoins, WOULD change every day.
You just argued against your own point. It's exactly what I've been saying.
Why should a bitcoin-based island big mac fluctuate when an aud-based australian big mac doesn't?