America can handle many, many more immigrants (remember, we don't have a natural-resource-based economy; more prime-age adults means we're richer per person). We should build more also.
Initial point was that China will close the power gap with us because their economy will eventually become bigger (making them the most important market, which gives them significant leverage in dealing with companies and other countries). The reason their economy will eventually be bigger even though they're much poorer per person is that they have a lot more people. The obvious way to counter that is to get more people. And their long-term population trend is down so we could turn things around again pretty quickly with smart immigration policy (including getting their best people).