Yeah. Their idea was to reduce the population until you reach a number low enough that is viable for the ecosystem. The population would then be "maintained" at a stable level. Most people would not be able to afford children.
You could make child-rearing unaffordable enough to require government assistance. Then that assistance could be limited by quota. There's a lot of things that could happen in the future. We don't really know. It's just a guess.
Another thing that In vitro fertilization solves is allowing for women to focus on their careers from age 20 - 50. If people in the future are going to live longer and be healthier, then it'd be feasible to have children (through surrogate) at the age of 50. If you'd already amassed enough wealth (or had privy to subsidies), I couldn't see why not...
If a woman has a child in her 50s she will spend her 50s (and maybe part of her 60s) taking care of the toddler and then raise them to be 10 years old. These are not easy years I can tell you that having gone through that more than once. And I was in my 30s and early 40s. Then, when she's in her mid-to-late 60s she'll have to deal with a moody and irrational teenager and try to make sure he/she won't screw up so bad as to ruin the rest of their lives. And she won't see them grow up to be independent adults until she's in her 70s. Is that what you envision yourselves doing in your last healthy years (and for many of us beyond that)?
We had our first kid in our early twenties, and by the end of the first two years, I couldn't for the life of me understand how anyone older than 30 can have their first child. Toddlers and babies are incredibly hard on the knees, back, shoulders, and your entire body. They're rough, active, and constantly demand various physical actions that will quickly exhaust anybody.
It's so strange having parents in our neighborhood two or three decades older than we were. It must be really hard.
I'm looking forward to my fifties where my eldest children are past college, hopefully giving me grandchildren to play with. I highly encourage young parenthood. It gets increasingly harder. The money part which so many get hung up on is easy. Babies are incredibly cheap, and your income has 18 years to grow.
Population growth is a solved problem as countries become wealthier. If you want to reduce population quickly, perhaps look at reducing the number of elderly people or reducing their economic dead weight cost (encourage euthanasia, export to cheaper countries, deathly games or sports, encourage pandemics, encourage depression, early payout for future euthanasia, et cetera). This could also solve the housing crisis (and housing is a driver for economic growth, and growth is really the main driver for global harm).
Direct costs for the elderly are currently ~40% of government expenditures in New Zealand and growing.
My guesstimate 23G$ ($==NZD in this comment) for superannuation/pension[1] and 16G$ healthcare costs[2] of 110G$ total expenditure for ~16% of population (789k of 5M in 2020, projected to 20% in 2030, and 25%+ eventually[3]).
That is ~60k$ per person. 50% of people retiring now can expect[4] to live 2 decades i.e. cost ~1M$ (totally ignoring discount value!).
> feasible to have children (through surrogate) at the age of 50
If you think it is okay to orphan 1 in 20 children, and that children don't have live grand parents, although maybe benefits of help with elderly care and passing on inheritance? 65 year old helping 90 year old is challenging and very common).
That would definitely reduce population (2 children per 2 adults reproducing at 50 has maybe 40% less total population count, compared against population count if reproducing at 25).
You could make child-rearing unaffordable enough to require government assistance. Then that assistance could be limited by quota. There's a lot of things that could happen in the future. We don't really know. It's just a guess.
Another thing that In vitro fertilization solves is allowing for women to focus on their careers from age 20 - 50. If people in the future are going to live longer and be healthier, then it'd be feasible to have children (through surrogate) at the age of 50. If you'd already amassed enough wealth (or had privy to subsidies), I couldn't see why not...