Links on research papers demonstrating significant correlation between infant-health/mortality-rates and paid maternity leave, here are the links to some research papers;
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7367791/

https://journals.plos.org/plosmedicine/article?id=10.1371/journal.pmed.1001985

https://www.mcgill.ca/newsroom/channels/news/longer-maternity-leave-linked-better-infant-health-259966

It's not just a question of financial means. There are other needs that the presence of the mother or a dedicated primary caregiver satisfies. Also, arguing that daily wage workers don't get the same benefits so let's remove the benefits for other classes too is going backwards. Progress would mean trying to get these benefits for daily wage workers too. The logistics of how to do this would be a different question.

Regarding the argument that declining birthrate wouldn't be a concern for the next 30 years, climate change projections also target the end of this century. But the whole world is making efforts in that direction, isn't it? It is not easy to reverse trends at such a big level and takes decades. Worrying about it when it already becomes a problem would be too late.