I think the birth rate problem will take care of itself. As there are less and less people the costs to the remaining will inflate, a declining workforce will be paying incrementally more and more tax, be required to work more hours and have a much later retirement age. At some point having children at 40 will become more financially untenable than in your 20s. This will mostly be achieved by giving financial incentives such as higher taxes, insurance rates and age limits on single and childless households, it won’t be phrased like that but rather involve being excused using cost of housing or the privilege. Personally I don’t think this birthdate decline thing is unwanted. It’s a lot cheaper just for industry to have less to output slowly over a few decades, than pay for the social changes needed to combat climate change.