So in case some of you Europeans were getting hopeful, no, mainstream demography does not predict that Americans are on the brink of extinction. Indeed, there's a rough consensus that US population will probably grow by about half again before levelling off. 

Nevertheless, like Jonathan Last, these same sources also anticipate an age-structure crisis — in the US as well as in Europe, China, Japan, and southeast Asia. Last's answer to the coming support-ratio crunch is for everyone to have more babies. 

It's a physical certainty that no population, of any animal, can increase indefinitely within a finite space. If you accept the proposition of a limit, whatever the number might be, then any country has to go through an ugly transition on the way to a no-growth scenario: lots of old people, not enough workers. Which is a giant drag for everybody. That means high taxes, restricted benefits, burdensome healthcare costs and economic stagnation — or, to use the latest fad euphemism for a Pandora's box of horrors no one wants to examine very closely, "pain". But unless we colonise other planets, this pain is inevitable — for every country, including developing nations. The only questions are when you bite the bullet, and at what population you peak. Making more babies now merely delays this reckoning, while raising the size of the population at which you stop.

Likewise, turning on the tap of immigration constitutes more mere delay. Last is certainly correct that America's influx of visitors-for-life has provided it with a younger age structure than Europe's, thus putting off our rainy day. But here's a little known fact: foreigners get old, too. So you either have a small, put-upon workforce toiling to support a bunch of decrepit white folks, or you have a similarly small, put-upon workforce toiling to support a bunch of decrepit Mexicans a few years later. Does it matter? Maybe it's better to get it over with.