>>107151884
That doesn't explain plummeting birthrates.
If anything, birthrates should be up if you get more benefits.
The reality is that those benefits don't actually go far enough to make it worthwhile. When a lot of the population were farmers or family businesses where the children could contribute to the income of the family, having more children is an obvious economic benefit.
But industrialization has increase productivity which has driven down the number of workers required, and often raised the skill level of the required workers (your kids would have to be in their teens to operate the equipment to be able to contribute to your family income), and at the same time education costs have sky rocketed (getting an education is literally tied to being in debt now).
To a lot of people, it is not economically viable to have 5+ kids. Or even more than two.