Highly developed capitalist countries tend to have low birth rates because people are alienated and atomized and having children is an untenable burden for workers who are already working themselves to exhaustion and barely getting by.

Yes of course it is an issue. When TFR reaches 1 for example, the next generation becomes half the size as the last one. You can't have a healthy economy with that. There's also every indication that more and more countries are reaching low fertility levels, and there's no precedent that it will reverse. Social democratic childcare policies haven't succeeded. Personally I think only socialism can achieve sustainable population levels now.

I think it's supremacists projecting their WHITE REPLACEMENT fears onto others

I don’t know

All I known are examples of Japan’s economic stagnation and the rise as France as the economic engine of Western Europe (highest TFR)

Your wealth or lack there of has no meaning unless there's people to perpetrate the capitalist cycle. Less people means less people without established capital. If your new people amount is lower than the dying out people amount you get empty housing which you therefore cannot pretend to have more value than it does. Furthermore, aging capitalists need a work force to take care of them. Humans live longer and longer, especially the rich. Without surplus of workforce it's hard to staff these positions. And so on and so on.

Makes sense, a lot of what’s written by the authors is hostility against austerity and they promote YIMBISM and more social spending