The problem isn’t the absolute population, but the structure of the population. By structure I mean the proportion of productive versus non-productive and the costs of caring for the non-productive portion. Elderly have much greater needs and expectations in their care, compared to children, and their needs grow with age rather than decrease. At the end of the elderlys’ dependence they aren’t productive members, but deceased so their is literally no future for society in them.
These things take care of themselves -- for example, we used to need 90% of the population working the land to grow enough food to feed everybody, now, it's more like 0.9%.
I swear, I could make a fortune selling sky-proof umbrellas to all the Chicken Littles out there.