A lot of people come to cities because there's an inducement: jobs, services, housing ... at least that's what they thought when they came. Not everyone can win that game. So how about some inducement to go somehere else? If a city is "unsustainable" then how about a golden parachute to somewhere that needs/wants them? Transportation, job training, moving van for your stuff, tax breaks ...
Yeah, I didn't think so.