Immigration reform hostage to welfare reform?

Tom Knapp complains about libertarians like Ron Paul and George Phillies who recognize that policy changes aren't made in a vacuum:

TK) It's a neat scam, this holding of one positive reform hostage, while around the corner your political enemies hold its counterpart hostage as well. You won't let your hostage go until they let their hostage go. And they won't let their hostage go until you let yours go [...] you've both successfully avoided responsibility. They've avoided doing something they don't want to do anyway, you've avoided doing something you claim you want to do but are looking for an excuse not to advocate. (TK

Knapp is right to lampoon libertarians who are so economically illiterate as to worry that America's current level of legal and illegal immigration is causing either 1) net negative economic impact or 2) a burden on the welfare system that is anything more than noise next to middle-class entitlements. However, Knapp is wrong to lump me in with libertarians who worry that America's welfare state is the primary obstacle to open borders. He should know from our discussion last year that I consider the primary obstacle to opening immigration is not the policies inside the border, but rather the policies outside it.

While most anarchopuritarians' policy prescriptions are unrealistically utopian merely about America, their stance on open borders is unrealistically utopian about the whole world. That's utopianism squared. Open borders between America and the rest of the world will only be feasible when the differences between America and the world are as small as, say, the differences between America and Canada, or between Alabama and Florida.

Libertarians just aren't thinking clearly when they condition open borders on replacing the welfare state with private charity. If hordes of poor immigrants would cause an intolerable expense for public charity, then they would cause a similar expense for private charity. To the extent that private charity doesn't replace public, it will result in massive downward pressure on unskilled wages and levels of mass squalor unseen outside the Third World.

Another way to state the argument is in a set of propositions which cannot all be believed consistently. For example:

1. Technology will continue to drive the costs (in money as well as social disconnection) of emigration dramatically lower.
2. Polities with significantly more economic liberty will continue to grow dramatically more prosperous than other polities, increasing their attractiveness as immigration destinations.
3. People living in such prosperous polities are reasonable to expect something better than the subsistence labor markets and ubiquitous squalor typical of economically unfree nations.
4. Charity (whether public or private) can and should maintain America’s current effective minimum living standards (which are extravagant by global standards).
5. Immigration of peaceful honest people should be unrestricted.

Even if one doesn't believe that current or prospective technological and demographic conditions don't combine with global disparities in economic freedom to make opening America's borders infeasible, it surely is not tenable to claim as a matter of political philosophy that a liberty-loving human polity should under all conceivable circumstances always allow unrestricted economic immigration. The concept of a "border" in these discussions is a demarcation between regions with different policies. It's just not intellectually credible to maintain that conditions and policies on opposite sides of a border could never vary so widely as to make unrestricted immigration infeasible. If your political theory nevertheless insists that unrestricted economic immigration is always a fundamental human right, then your theory is just plain broken.