Tyler Cowen says that advocates of open borders are doing the cause of less restrictive immigration laws a disservice. I think I disagree. His view seems to be that if only people would keep quiet about the strong case against immigration restrictions, then nervous and risk-averse people would stop being frightened by the specter of truly open borders. It seems to me that the specter of truly open borders is such an obvious specter for nativists to raise that proponents of more liberal immigration laws had better have something sensible to say about it.
What I think ought to be said is that open borders is an ideal that the world ought to work toward. It would not be practical for, say, Denmark to suddenly and unilaterally lift all restrictions on migration to Denmark.
But a world in which every country except Denmark retains very tight restrictions on immigration and then there are no limits on migration to Denmark is not a world of open borders, it's a world in which Denmark has adopted a very idiosyncratic immigration policy. A world of open borders is a world in which it is easy for a person to move to Denmark or Japan or Germany or South Africa or Chile or the United States of America or anyplace else. A world in which people in general can move fairly freely as a general matter. Now of course in such a world you can raise the hypothetical "well what if everyone decided one fine Tuesday evening to move to Denmark?" The answer is that you'd have a mess on your hands. By the same token, if every current resident of the United States were to decide this evening to try to move to Connecticut you'd have a mess on your hands. But avoiding the scenario in which tens of millions of Americans suddenly try to enter Connecticut does not, in practice, require legal restrictions on who can move to Connecticut.
It's completely reasonable for people to be apprehensive about sudden changes and about unilateralism. But those kind of concerns are perfectly compatible with accepting the moral logic of open borders and hoping to see enough liberalization all around the world that some day law-abiding people can move around freely. In the interim, it's obviously not possible for democratically elected public officials to simply ignore the concrete interests of their constituents when making immigration policy so it's natural to look for liberalizing measures that boost native well-being rather than just liberalizing willy-nilly.