Countries would balance out – badly governed countries would empty out, well governed countries would have higher populations etc,.
How is the UK going to find homes, infrastructure and jobs to absorb the potentially almost infinite number of people who could come here? Can you imagine the chaos? How is the government supposed to ever plan ahead?
What you neglect to say is that well-governed countries would fill up until they become as uninhabitable as the third-world countries people are escaping from. Do you want that?
I painted this, call “Borderline Madness” to show the stupidity of artificial borders drawn across a single entity purely for the sake of borders.
Borders are no more ‘artificial’ than laws against murder or the existence of money. Consider that they can be no true freedom without the laws that flow from a political community. There would only be chaos and anarchy instead. Without states how could we have political communities able to establish laws? With total freedom of movement these states would be more like hostels filled with transients. What are the consequences for the political community?