> I don't know if "anti-natalist" can be so simply placed on the conservative vs. progressive axis.

That was kind of my point. I doesn't belong on that axis at all, especially trying to transpose a comment from decades ago onto the current political dividing lines between "left" and "right" when so many ideologies have shifted sides over that period.

> Worrying about overpopulation means worrying about the quality of life of future generations, whereas "natalism" à la Musk is basically worrying about how they can keep making themselves richer, while not giving a damn about what happens to the world or humanity afterwards.

One can be natalist or anti-natalist for a variety of reasons. There's no one ideology that leads a person to each conclusion. People have come to (anti-)natalism from both humanist and anti-humanist arguments (and a variety of other arguments).

> So Turner's concern about population growth, if not necessarily progressive, strikes me as very fitting and in line with the humanist, philanthropic positions shown by the other points mentioned about him.

Again, avoiding the left/right/conservative/progressive labeling as those terms are dynamic over time. He's essentially a Malthusian for humanitarian and environmental reasons. Which makes sense, given his age. He'd have been in his mid 20's through the 60s and that was a fairly popular viewpoint at the time.