The parable needs at least one bit of extension:

It costs the inhabitants nothing to move to another island, especially in light of the fact that resources for everyone have essentially dropped off the map. They might not get the same kind of resources, but they'd have access to more neighbors who might appreciate their lawn or the way they paint their house.

After all, they can no longer make a living on the island they were previously enjoying so they might as well not make a living in a pleasant community.