I too am curious as to why that is the feature they are concerned about.

As I recall there was a UN survey that found financial security was the most common reason for adults across the globe to choose not having children.

Income levels are highly dependent on the degree of stability and monthly expenses that households can expect.

It's probably also worth mentioning that nobody predicts birthrates to fall indefinitely - the human race isn't just going to die out because multi-child families are less common. Instead we expect that as affluence becomes more evenly distributed, the whole thing will stabilise at a lower population (and ideally one that leads to less competition for natural resources)

Outside of "great replacement" types who are worried about the continued supply of purebred aryans, I'm not really sure why anyone would be concerned by this