May be in the future, when a country have all power over the others; or all the countries have the same power.
To put a proper criticism of the current system together would take me pointing out premises and assumptions that people do not even know they have. I'd have to start by sharing some history going from the Peace of Westphalia to the end of WWII to talk about how the current notion of nations and borders developed.
Then I'd put my case together that I think there's some fundamental human rights that I reckon everyone should have, that no local government should have any jurisdiction over. One of those, of course, is full mobility like I put above, but also free speech, free assembly, and so on. Then I'd mention that localities could make local laws to handle things a bit different and specific to the particular place.
Finally, I'd take some guesses about how this would happen, where some set of armed group(s) or allied nations would declare a global constitution that all people had a right to, and a slow change would take place. Or perhaps decentralization of information makes enforcing borders effectively pointless. Or maybe warfare evolves even further away from conventional warfare as seems to be happening now.
Who knows? It's a complicated mess of a topic. I just think it'd be a really nice thing :)
-- Albert Einstein
There is nothing that pisses me off more than one human standing in front of another saying they can't go somewhere.
This planet belongs to none of us.
As long as somebody does no actual harm to it they should be free to go anywhere. Borders are one of the worst inventions ever and there ought to be a universal human right associated with your idea.
Now it has quietened a bit, I suppose who wanted to go went in that first wave.
Now, suppose we opened the borders to India, Brazil, Mexico, Russia, Pakistan, etc. The Mexican and Brazilians will probably go to the US rather than UK, the Indians and Russians will probably want to come to the UK rather than US. India is 1 billion however, Russia 300 million. If a fraction of these people came to the UK the country would be flooded and go into a period of complete transformation and utter confusion and most probably civil unrest as the primitive territorial instincts take hold. The Indians will then demand that their culture is respected and therefore push for laws rather than integrate and do what they do in Rome.
In principle it is a noble idea. That is why although the EU project has its downsides, most people think it unthinkable to leave the EU. We like being able to travel freely in this continent, to allow trade to be completely unrestricted, but we did not sit one day and say, hey let us open our borders to all the EU. It has instead taken about 60 years for this project to evolve to what it has and still not all of the geographical europe is within the EU. So perhaps we should gradually have an EU style project in a world scale. That is, I suppose, what the UN was meant to be, but it has effectively failed. Maybe now we should try and set up institutions and a judiciary for a United Western World, whereby people are able to move freely within Europe, US, Canada, Australia, New Zealand and then gradually incorporate Russia, India, Brazil, etc.