The solution is the successful completion of the Gilgamesh Project.
No one is going to be able to get TFR back to 2. That means for every woman who doesn't have kids, you need another woman to have 4 kids. That will never happen again -- it's too easy to not have kids and borderline impossible to have 4 kids these days.
The only solution is to make people live longer, much longer, and stay in working health, so you don't constantly need a large amount of births to counteract deaths.
Human civilization has about 50 years to figure out amortality before our society gets too old to make breakthroughs.
IMO solution, without depending on theoretic breakthroughs, transiently authoritorian, with some historic precedence / existing state institution / cultural memory is birth quotas + state orphanages to sustain demographic pyramid. PRC in 90s with mass migrant workforce transition had something like 20m boarding school students, 10-20% of student body depending on urban / rural. State was already raising a lot of kids, state can again, but need surrogates obligation to generate bodies.
AI math that I'm not going to double check but assume correct:
- Goal: maintain 800m demographic pyramid
- Assuming: 80 year life span
- need: 10m annual births
- Assume positive policies: raise TFR to 1.5 = 7m annual birth
- State "incentivizes/compels/coerces" 3m births for orphanage system
- Orphanage system size raise 3m x 18 years = 54m, ~3x peak PRC boarding school in 90s when PRC had 5%/15% (ppp) of per capita resources
- Throw in some genetic modifications to bias female birth selection to increase available surrogates (i.e. 60% female = "on'y" 40% particpation rate)
- 5000 usd per kid / year (5x gdp percapita of 90s) = ~250B program / 1.5% of GDP.
That's on paper workable state demographic management system. How you incentivize women to surrogate as social obligation and balance men social obligation (i.e. need to give up same amount of years/opportunties if not more) is up to details. Something like women turn 19, free tuition / boarding / i.e. best conditions for teritiary / discount / priority / upgrade on everything, spend 6 years vs 4 for schooling. Squeeze out a baby while pressure low, career effect minimum, get to enjoy years of tier1 service (i.e. hsr upgrades to first class) per child to incentivize / hook when women young / have access to least resources. Activate some maternal attachment - some single moms who don't want to keep babies experience attachment shift, more likely to have kids later. Men can make up geriatric end of social service spectrum - have to spend proportional time/energy taking care of elderly, every 6 months of school, men spends 3 month contributing to senior care. Implementation phase also potentially stretches out workforce entrance = mitigate ongoing issue of accomodating for millions of youth teritary. Basically equalizes career penalty for women so both genders start workforce same time. Also need to be extremely punitive towards those who don't contribute to system.