High numbers intrinsically defeat immigration, both by making ghettoes viable and by creating political pressures for multiculturalism. And if the problem is that the means of assimilation are broken, the obvious answer is a moratorium until they are fixed. Immigration will force us to accept social mores we abhor. Moslems, for example, will vote to allow polygamy. Immigrants also entangle us in foreign policy commitments that are not in the national interest, and threaten our existing ones.
Immigrants massively vote Democratic, worsening all of America's political ills.
The simple fact, which nothing can ever change, is that more people means more environmental damage. Even if regulation and technology mitigate this, more people still means more damage than with less people. If we bring in more people, this means existing Americans have to reduce their standard of living in order to make room for foreigners to come here and pollute. This will be the pattern for the future on other environmental issues. Immigration-driven population growth drives sprawl, traffic congestion, and the costs of suburban growth.