The problem with looking at a complex system full of inter-dependencies is that it always yields an incorrect conclusion. One of history’s recent examples is how the Hunt brothers failed to think carefully about supply and demand. They attempted to corner the world’s silver reserves, failing to understand that as the price of silver rose (due to the Hunts buying up silver) this would incentivize people to sell their family silver: not bullion but teapots, necklaces, and rings. If the Hunts had possessed hundreds of billions of dollars, they’d have ended up spending 80% of it trying to acquire the last 20% of the world’s available silver. And this would merely have resulted in mining companies extracting even more silver…
Now, moving to the question of open borders we need to ask: why do so many seek to flee the lands of their birth? It’s simple: grotesque misgovernance. Most countries are even more corrupt and dysfunctional than the USA, which is marketed abroad as the Land of Opportunity. But as people leave a country, the despots in charge discover that without people they’re presiding over a hollow shell. Over time this increases the value of the average citizen which results in better governance. Which in turn reduces the number of people seeking to go elsewhere.
Of course this process can take 30 years or more but it does happen. When it does, many living elsewhere then opt to return home. The British have seen this with Polish immigrants, for example.
So: never treat dynamic systems as static systems displaying a single variable.