[quote=deadzone]Can you name a single developed country with open borders in terms of immigration policy?[/quote]
Europe has open borders. EU members can now travel between many sovereign nations without a passport and have the right to live, work and receive welfare in those countries. There are restrictions imposed at the local level. You find many young eastern Europeans working in western Europe. They are enthusiastic and willing to take jobs often the reserve of previous immigrant waves, who themselves should move up a rung.

The migration flow is two way. Those seeking a lower cost retirement migrate eastwards. In theory, Europe should eventually be the melting pot that is America. In historical terms, it is already a melting pot. There is a downside. Locals will become embittered by what they see as an invasion, displacing locals in the job market and exploiting welfare. Criminals can also move more freely.

I see no reason why borders could not open in the US with its neighbors, if there is a common interest and mutual benefit. Economic disparity may rule that out for the time being, but both Mexico and Canada are doing well. I’d think Mexico could be a fantastic place to retire.