4. Note: If you buy this argument, you should be similarly afraid of economic development in the Third World. So rather than opposing immigration, you should oppose economic progress in general.
5. The heart of my reply to the environmental objection: the Environmental Kuznets Curve. Quick summary of the empirics: Moving from low to middle income increases environmental harm, but moving from middle to high income reduces environmental harm. So environmentally speaking, the best thing for the environment is to move from low to high income as quickly as possible. And liberalizing immigration does precisely that! Indeed, immigration lets people leapfrog straight from low to high income without even passing through middle income along the way.
6. Caveat: Standard measures probably overstate environmental quality in low-income countries by ignoring noxious low-tech pollutants like animal and human waste. So leapfrogging straight to high income is even better than it looks.
7. The Environmental Kuznets Curve works through multiple channels: consumer demand (richer people want greener stuff), norms (richer people care more about the planet), and regulation (richer countries can better afford the economic burden) being the most obvious. But we can safely liberalize immigration without pinning down the precise mechanism.
P.S. Any related topics you think I should address?