> We also don't necessarily know how more people raises GDP per capita. Maybe it's a situation where having basics in place fosters more people who are healthier or something and maybe in some countries that pattern is not what's happening.

Is there some type of underlying assumption that goes like this: "more people ==> more people participating in the market ==> more consumers for your product"?

In that case, to add to your point, I think "more people ==> more people participating in the market" is problematic. It is perhaps true in the following grotesque sense: "the more 'shit'(objectified, person/product) that's produced and thrown at the wall, the higher the chance we find some(thing/one) that sticks to the wall (meaningful market participant)", but that grotesque perspective ignores that the "things that doesn't stick to the wall" may unexpected systemic effects that reduce market size (e.g. greater depression amongst the wall-clingers, due to the suffering they see around them, harder to elicit feelings of shared community between all pieces (whether they clung to the wall, or pooled to the bottom) because they know they are disposable: i.e. loss of trust in humanity), etc.). It reminds me of the country my parents came from.

In general, people don't like to believe that every human being has great potential (and that most people do not get to reach this potential, for a variety of reasons, many systemic) because then actions that negatively affect other people's potential, even in small ways, have greater-than-expected negative externalities. That's bad for business; while any benefits reaped from the other perspective (in building a stronger market in the future) can only be reaped in the future, and therefore worthless ("a society grows great, when old persons plant trees whose shade they will never get to rest under...").

So it's easier to think that people are born with a fixed maximum potential. Those that can reach it, will most likely reach it, while those who do not do so well likely did not have that potential to begin with.

Based on this, it makes sense to have more shit thrown at the wall to see what sticks.

I think also many of the fertility arguments are based off of "undesirables have high fertility rates", therefore, "desirables must increase their fertility rates!". Put differently, it's not always a call for "greater human fertility; without any qualifications", but a dog-whistle with some uncomfortable qualifications. (c.f. the discomfort of having "desirable persons be in relationships with undesirable persons").

Related to this is the perspective that desirability is due to the existence of certain historical traditions, therefore, desirable parents have a better chance of inculcating desirability into their offspring, than society has of inculcating desirability into undesirables through lost causes such as education or inter-cultural understanding.

Not related to the last, but other similar ideas are: "If we do not have enough humans, who will do the dirty work? Could end up being us!" or "Children are the machines who support us when we grow older!". Anyway, I'm getting bored, so will stop here.

To sum up: whenever entities present "we need more fertility" arguments, it's rarely because they want to explore reality for everyone's benefit, and often because they have certain pre-decided perspectives on how their future (not their children's) may be safe-guarded.