David Frum, reviewing Mark Steyn's book, makes a great point that's been weirdly ignored during the contemporary fad over demographic fear-mongering: "Demographic trends have a surprising way of reversing themselves with amazing rapidity. ..."Of course, Yglesias likely knows the same thing is true regarding climate change. In the mid-seventies, after global temperatures had been cooling for more than three decades, global cooling was portrayed as an imminent danger of catastrophic proportions. Then global temperatures began rising and have continued to inch upwards over the last three decades, so cooling stopped being sensationalized (for the worst, since global cooling presents a far more threatening existential threat to humanity than warming does) and warming became the story. Just because something has been capricious in the past doesn't mean it's prudent to write-off thinking about it in the present as a waste of time.
Demographic trends, like climate trends, have in their histories plenty of unexpected pivoting. But global temperature tracks rigorously with solar activity (CO2 emissions do as well, but trail temperature changes by several hundred years). What to do about the sun? Ideas are out there, but the subject is more arcane than fecundity patterns. Most people have some firsthand experience to bring to the table with respect to the latter.
Outside of Israel and the US (barely), the developed world is procreating at a rate below replenishment. Unique to most of human history, today affluence and fecundity are inversely related, accentuating the wealth gap and conceivably creating a dysgenic effect. And the full effects of birthing pattern shifts are felt generations after they begin, so it's hardly too early to be thinking about possible consequences and strategies surrounding what might be a societally-threatening sustained birth dearth.