If people have to ask themselves why they should have children, and start doing economic calculations, it’s already over. It means your civilization has reached a point where it can’t even justify its own existence. For pretty much all of human history, having children was simply something that just happened. People got married (usually), had sex, got pregnant and had kids. That’s it. If your society is having debates on how to increase the birthrate, you’ve already lost.
Of all the socioeconomic factors that led to the collapse of birth rates in advanced economies, I would argue none was more impactful than the expectation for women to join the workforce and work until retirement like men. Women joining the workforce then led to women being expected to have a good education too; after all, if you join the workforce, you at least want to have a decent job with good wages, and a good degree is certainly helpful in this regard. This led to women delaying family formation more and more, and for an increasing number of them it meant having no children at all.
This is why all the efforts by various governments around the world to deal with sub-replacement level birthrates will fail, even in totalitarian states like China. Because despite their proclaimed social conservatism, neither China, nor Iran, nor Hungary etc are willing to get women out of the workforce. Egalitarianism between the sexes has been so ingrained in the collective consciousness of advanced countries (including ‘authoritarian’ countries), that the idea of women not going to university and doing full-time wage labor is just inconceivable to them. Rumania’s policies under Ceausescu in this regard for example were totally schizophrenic: on one hand they banned contraception to get women to have children, but on the other hand they still expected them to work outside the home (as was usual for socialist countries).
There simply is no policy solution to this – unless you’re willing to use the full power of the state to enforce an ideological commitment to women as homemakers and mothers. That’s what the Nazis did in the 30s, and it’s probably the only example of a society managing to significantly increase its birthrate by decree without needing to ban contraception.
I think the only way out of this is through. Advanced societies will simply have to shrink, until the only ones left are the descendants of those who wanted to have children, despite all the incentives not to. There still are people who have children, some even more than two, and not all of them are poor people. As someone else said, we are currently living through a genetic bottleneck, possibly the biggest bottleneck since humans first left Africa. Who knows, in the end it might be for the best, and whatever comes out on the other side of this bottleneck might well be better than what came before.