If people are free to leave their current polity, then governments still need to compete for people wanting to live within their domains and are subject to a fr

If people are free to leave their current polity, then governments still need to compete for people wanting to live within their domains and are subject to a free policy market. In other words, the political pressure and good economic policy you're referring to are contained within a free market, not the other way round. In fact, even if you're arguing for a state's right to ban emmigration, that's just an additional difficulty to voting with one's feet. People still left East Germany despite it being illegal, because West Germany was outcompeting it in a truly unconstrained market of where people wanted to live.