The interesting thing about cities (not city-states) is that while they are clearly entities, they do not have borders. They bleed and blur around the edges. It’s very easy to come and go. It’s very easy to move there or to move away.
In a political context where every now and then people like to trumpet post-national politics with a rise in urban power to match the drop in state power, this is very interesting. Once you have crossed the borders of the state (the ones that aren’t police states) you get free reign to come and go as you please from place to place; something that can have wonderful or disastrous consequences for the health of an urban environment and the people left behind.
You can see it in a tour of the nation state that is a continent. Consider the fear that the people of New Orleans would simply refuse to move back after Katrina. Consider the slow disaster in Detroit. Consider the millions who move to New York or Los Angeles in the hopes of making it, and the millions more who leave. Consider the suburbs and bedroom communities of people who want to live near a city without living in it. Consider that it’s easier to move from Miami to Seattle than it is to move from Buffalo to Fort Erie.
The result is that at the same time that cities are growing in power and making independent deals with other cities (there’s a lot of that around the Great Lakes region) that they have a population that’s shifting and amorphous. You can try weird experiments in governance and if people don’t like it, they can relatively easily just go somewhere else, often within commuting distance.
In some ways this opens up a lot of exciting possibilities but in other ways it weakens civic engagement. Sometimes reforms are necessary and painful. A lot of the problems we’re facing are at a scale that’s larger than the city, but that if cities are the new seats of power that we must deal with tools at city-scale. If your opposition to a policy means you just move a county over to avoid it, policies are harder to enact. If you don’t want to pay taxes, you move to a bedroom community and rob the the infrastructure of life-giving dollars. But the problems don’t happen county by county. They happen all over.