TFV said in a comment that the device would keep adjusting life expectancy with new medical advances, etc, so if we suddenly cured cancer, for example, the average cut-off date would go up for everyone to what it would be in a world without cancer deaths. So I think we would suddenly see a whole new human drive towards charity, and health care for everyone, etc. because the early (non-)death of people in other towns suddenly DOES affect you. I think that might do humanity some good, actually.

Without that, I agree that it might end badly.