>You can either have open borders or a welfare state, but not both. Otherwise, what is to stop millions of poor masses from migrating to the country offering the best welfare and driving down the wages for locals (by, say, willing to work for a lower standard of living)?

What stops this from happening in the Schengen Area, where some countries participating have welfare states and some (I believe) do not? Genuine question - I've heard other people make the same argument before and it seems like we have a pretty good real-world example to either verify or refute it.