I have trouble understanding how a smart guy like Yglesias manages to keep going further down this path, but when I've said that he prefers society to always have more younger people than older people, I meant it as a somewhat joking criticism.
The joke's on me, because he's pretty literal about it now as a path for growth:
[If immigration and lax land use regulation prime a state for population growth] then you don't need any particularly optimistic beliefs to see that the state is primed for certain kinds of investment. We're going to need new houses for these new people in the short-run, and we'll need new schools & hospitals, new car dealerships, and new highways for them in the medium run. So we're investing. And with that investment happening we need new Whataburger franchises and new H-E-Bs and probably new power plants as well. And now suddenly we're on the high equilibrium. We need more accountants and more wedding planners, we're going to need some fancy restaurants, we'll need hotels, we'll need more of everything. And since we'll need more of everything and the price of new homes will remain moderate, we'll expect the population to keep growing as people from around the country tend to move here in search of work.
As for how long that all is supposed to last, he's silent. Ironic that the very next (not so good) post was about economic bubbles, so he acknowledges issues of unsustainability, while missing his own huge blind spot.
And yes, population growth can help economic growth, but it's unsustainable in any number of senses of the word.