Earlier quoted context omitted.
For a migratory species it is literally inhuman to erect a border. Borders should be jurisdictional boundaries but should be permeable like the states in the US. The rest of the tribalism BS is just that.
Humans stopped being migratory about 12,000 years ago. You probably don't want criminals to freely move into your country. You probably want a country that gets funded by taxes (both short term and long term). No country in the world maintains open borders. Everytime there has been mass unorganized migration it has been a humanitarian disaster. Platitudes only work on paper.
Criminals move between states, but it's not a huge problem. The states can share information and allow joint action. It may require more effort to set up between nations, but it's a common thing today.
Taxes work fine. If you're a resident of a state, you pay income tax to that state. So we obviously know how to find out residency.
>Everytime there has been mass unorganized migration it has been a humanitarian disaster.
If borders were to become easily crossable (not necessarily non-existent, but just a checkpoint to count incoming/outgoing and stop smugglers) in a day, then the next day would see mass migration in so many directions. But after that first wave calmed, I wouldn't expect mass migrations except those following humanitarian disasters. Three had migrations we see today are caused by disasters. Even when the emigrants experience terrible conditions in the receiving country, I imagine it's because their previous life was shattered and they had to quickly flee. It's not a fair comparison to people who would migrate if immigration controls were removed.