Gallup looks at migration wishes:
If all of the adults worldwide who tell Gallup they would like to move to another country actually did so, the United States could see a net population gain of 60%. Several other developed countries, such as Singapore, however, could be even more overwhelmed with migrants because of their smaller relative current population. Mexico, on the other hand, could potentially see net population losses as high as 15%.
The ability to attract immigrants is not all good, of course, but it does speak to the country's capacity to regenerate itself and stave off a decline in population. America's two major great power rivals - China and Russia - can boast of no such attraction.