A simple argument for open borders or at least nearly open borders using global distributive justice.
Rich or developed countries should help poor countries like how welfare state helps poor people.
Rich person has an obligation to help the poor person [This obligation is not too demanding. Think of just some form of moderate taxation. Rich people can still enjoy their luxuries. They just need to care sufficiently for the poor.] with ways like offering decent jobs, or money (see ‘GiveDirectly’ or ‘New Incentives’ charity).
Adding numbers does not make the obligation evaporate, for example, 5 rich people will still have obligations to help 5 poor people. That is, rich groups should help the poor groups.
From 2, rich countries have obligation to help poor countries.
There are two ways to help - a. helping the poor countries using large investment projects (or some sort of effective wealth or capital transfer) in those countries. Or b. allowing poor people to move from poor countries to richer countries, that is, labor transfer and some sort of welfare state help.
Labor transfer is easier (that is, more efficient or effective) than capital transfer.
So, Open Borders or Nearly-Open Borders would be better.
1 is easy to justify because all normative ethical theories or views, except deontological Hoppean/Rothbardian/Randian libertarianism and Narveson’s contractarianism, support the obligation of rich to help the poor. Deontological libertarianism of Hoppe/Rothbard/Rand is false. And so is Narveson’s contractarianism.
Contractualism, Utilitarianism, any welfarist consequentialism, virtue ethics, moderate deontology all say that rich people should help the poor.
2 is also easy to justify because adding numbers does not magically destroy value or obligation. Some might think that what if obligation is too demanding (due to large amount of poor people and very low amount of rich people) and also leads to less ambition to create good stuff that leads to reduction of poverty? Then the answer is - as i said earlier, the obligation is not too demanding because rich people don’t need to donate literally all wealth or nearly all wealth. They just need to sufficiently help the poor. The “sufficiently” can be intuitive. For example, Norway, Sweden, Finland, etc. have interesting levels of taxation that cause decent amount of wealth transfer from rich to the poor and keeping inequalities controlled. The amount of wealth transfer can be modified if it is too much or too little.
3 is straight forward.
4 is justified because there can be either rich countries send people and money to help the poor countries (movement of capital) OR rich countries allow poor people to move from their poor countries low amount of capital to richer countries with high amount of capital (movement of labor). There are no other possibilities (unless someone considers murder, starvation, etc. as “help”, but that is psychopathic).
5 is easy to justify because large investments, infrastructure and institutions building would require much much much more work by the richer countries compared to just opening the borders for the same benefit (that is, increased total wellbeing). And rich countries would have to also fix the civil wars, but by opening the borders, you can simply allow people to move and no need to fix civil wars or ethnic tensions because maybe one ethnicity can go to one rich country and another ethnicity to another rich country, and that would also mean that those ethnic tensions are healed with the wheel of time. Of course, it would be better if everybody went along well immediately :D .
6 straightforwardly follows from 5.
when I was a kid we and we had arguments we would draw a line in the sand and say step over that line and i'm going to knock the smelly stuff out of you.
Borders are lines in the sand. Even some islands have lines in them (Hispaniola, Ireland) others are island collections (Philippines, Japan); others can't even claim their natural water border as belonging to themselves (Azores, etc.) Borders are leftover empires divisions of their own unmerited spoils of conquest. Including the borders of the US and European nation states and the many 'third" world states created from the ruins of those despoiled borders.
A simple reason for open borders, and you actually mentioned quite a few good ones---are they are merely childish bully dares challenging people to cross or not cross non-existent lines that the bullies have drawn to take possession of those they can define as their national serfs.