Thanks for taking out the obvious straw man with "(excluding border cases of known criminals, etc...)".  Yes, a defence preferably with some data would have been good.<p>Yes, I do believe that unrestricted immigration would not have a substantial negative impact on quality of life for the resident population.  And would have an enormous positive impact on the migrant population.<p>People from the poorer parts of the US are already allowed to migrate to the richer parts.  Why stop there?  Also as poorer nations joined the Schengen-Area (related to the EU) there have not been qualitify-of-life-decreasing mass-migrations.<p>You may argue that there can be quality-of-life degradations when the immigrants are not properly integrated.  But the US seems to do a pretty good job in this respect.  (My home country, Germany, much less so.)<p>Sorry, I do not have enough time to write down a real argument, but if you are really interested, I can perhaps prepare a blog post or so.