Private business was just one of several examples I gave to illustrate principles which hold across a wide range of scales of human organization.
It is straightforward to verify that they don't stop holding at nation-scale. A good place to start is Lee Kuan Yew, who's both one of the only leaders in history to preside over the entire transition from Third World to First World for millions of people, and an architect of what's currently the most open immigration policy among Chinese-majority states. Singapore is a multicultural place whose government is widely acknowledged to have one of the best technocratic track records in the world, and they spent considerable effort on trying to get immigration policy right, iterating through alternatives while deliberately taking a popularity hit; what were their conclusions?
Or what are the odds, under your stated worldview, that two of the closest things to actual open borders existing in 2018 are
(i) the Norwegian archipelago of Svalbard, which lets almost anyone stay as long as they can support themselves ...but ranges from 74 to 81 degrees N latitude, and
(ii) the United Arab Emirates, which has more than five times as many expats as actual citizens, the largest number from India ...but ruthlessly maintains a two-tiered society, where even people who were born in the UAE and have lived there their entire lives are exceedingly unlikely to be granted citizenship unless they are Arab?
Contrary to claims that nothing like open borders has ever been tried, the existence of cases like this make it abundantly clear that this area of policy space has already been subject to substantial exploration. And every surviving result has an obvious and unusual force that does something similar to what traditional immigration controls accomplish in most countries, such as Svalbard's -40C winters. This is, to put it mildly, not a likely result if traditional immigration controls no longer have a useful function.
