I am not sure if this is a serious question. Generally the idea of a common travel area makes sense if the countries at a similiar economic development (like the Schengen zone). This also means that the training skills of the people are not too differend; and this prevents all kind of social problem you would get otherwise.
It's an honest question. I just don't see that question ever being asked by anyone, to me they don't make too much sense. I understand there'd be a period where things would get messy but won't we eventually reach an equilibrium where everyone is where they want to be, no resources wasted on borders and immigration and everyone just travels and works freely everywhere?
If the argument is "everybody thinks that" then that's the form of learned helplessness I talked about, which is why I asked here, to hear some pragmatic reasons. The only argument I hear being repeated is the same vague "otherwise it would be bad" from one side and the "borders are imaginary" from the other side. To me it seems we just keep them because it's been this way since the Roman empire and noone knows really why they're still there.
On the other side of the argument, yeah many things are imaginary but that doesn't invalidate them. A football team is an imaginary group but serves a purpose, to play a game. Country borders as well separate cultures, food, music and people with different languages, that's fine, these make sense, but why isn't free movement allowed? Polish didn't move to Germany en masse nor Germans to Poland while there is no border control and there certainly is economic inequality between these countries.
On the world scale Poland and Germany are in the same economic tier.
The part that it will be messy, was actually what yourself said (so I didn't think you need evidence for that).
Well, one can also look at other models historically, if you think of a border as demarcation of the influence of a state. Historically China didn't have well defined border, the further away civilisations were vassal states which had to pay tribute (but received gift of higher value than the tribute, so the real tribute was acknowledgement of power).
Another example would be afghanistan/Pakistan. The border is very transparent and at least in Afghanistan there is not a functioning government, but war lord tribal areas.
But this is about border of state demarcation. The idea of people moving to other places is generally a new one (apart from earliest pre history or when masses do this, like in the migration period) new in the sense starting with colonization of the US.
