"...Lee and Miller estimate that the country would need to admit an additional 5 million immigrants per year, quintupling the current level of immigration, in order to achieve long-term balance in the Social Security trust fund. A recent report from the United Nations Population Division reached a similar conclusion for European countries, announcing that even much larger migration flows than are currently permitted would not counterbalance the effects of population aging.""To maintain the 2000 ratio between the working-age population (people between the ages of 20 and 64) and the older population (people ages 65 and older), the United States would need roughly 95 million more working-age persons in 2025, in addition to those already expected at current levels of immigration. In other words, if the entire working-age population of Mexico were to move to the United States in 2025, there still would not be enough people to restore the old-age dependency ratio of 2000." -- Source
If we have to repopulate via immigration, I'd rather see people come from South America than from Muslim countries, as is the case with Europe.
A far greater threat to this country than immigrants -- which nobody likes to talk about -- is the depopulation of its rural areas.