We are already seeing a growing appreciation for practices like ethical polyamory, open marriage, etc today, coupled with the growing depreciation for things like real estate while the divorce rate tells us traditional marriage is becoming untenable.
The polyamorous communes of our progressive descendants may be more robust emotionally and economically, as they assume those functions of the nuclear family without the constraints of "traditional" two-adult family units (cheating, disinterest, work/life balance, max 2 earners, etc). The traditional perspective believes it has a monopoly on social harmony and fertility because it adheres to strict, heteronormative, and patriarchal rules that appear to have "always worked"; the wager our this collective polyamorous future is to recreate those "village" conditions, even more primordial than the nuclear family
While you’re right in that nuclear families aren’t ideal, the more fertile subcultures are those where extended families are valued. And these cultures are, by and large, religious and culturally conservative. The only issue there is the tendency for the children of those cultures to assimilate into the progressive mainstream; after all, that’s what happened to us.
We've created a society where people who recklessly reproduce with no concern for the consequences or anything except for themselves, will not starve and will mostly go unpunished for their awful decisions. At the same time those in the middle who are more educated and socially conscious tend to be much more cautious and judicious with reproduction. Even those on the top who are capable of 'responsibly' reproducing are increasingly choosing not to. This is going to lead to an interesting future...
I read that more as "have defused the population bomb". Cultures that have less children have more wealth to pass on to their children, making each one more likely to survive. The rest of the world having tons of children is likely to face shortages of land, water, food, health care... etc. This leads to all sorts of instability, which I'm sure you can imagine.
The thing is, liberalism has a lot of memetic fitness too. Classical liberalism has been devouring the world for the past few centuries, and while it occasionally suffers setbacks, it's done very very well against kings, dictators, popes, shoguns, and the like trying to hold it back. Just because liberalism is in a decline now doesn't mean that it's lost its power.
Rich liberal societies consistently reproduce at less than replacement. This is obviously unsustainable; a population that doesn’t reproduce fast enough to replace itself will go extinct. Obviously unsustainable growth isn’t a great thing either, but as long as the rate of growth is low enough for technology to keep up, the earth can actually sustain a surprisingly high population in comfortable conditions. The limiting factor turns out to be energy.
Why on earth do people equate negative population growth with extinction? Populations of most species rise and fall all the time with availability of resources and amount of predation. This is Ecology 101 stuff. I feel like most of the people who argue this have some sort of deep-seated fear that the world with run out of white people and God forbid non-Caucasians take over the world. But societies reproduce memetically, not genetically, so it doesn't really matter about local reproduction levels -- as long as you have immigration, a society survives.
Agreed that energy removes a lot of barriers, but not all of the limits. Earth is still a constant size, and having an ecosystem is helpful to us. But with enough energy, other planets start looking attractive.
It’s also abundantly clear that human population decline has absolutely nothing to do with availability of resources, because human populations with more resources are paradoxically less fertile.
Speaking to global warming in particular—once a single technologically advanced country figures out how to cheaply extract CO2 from the atmosphere, either through a more efficient means of doing that or through a breakthrough in energy production, that country can unilaterally solve the problem forever, even with continued use of hydrocarbons (since they can operate a closed loop of reextracting used hydrocarbons from the atmosphere). The fact that we aren’t there yet means that population growth outstripped technological growth for a long while, but in the long term, it’s a solvable bottleneck, just as the low agricultural output of much of the world was a bottleneck solved by Norman Borlaug and the Green Revolution.
I feel like you're borrowing trouble, though. Populations with more resources are less fertile, for now. But unless and until global population falls below two billion humans, it's not really our problem to solve. Any number of things could change socially by that time. In the meantime though, dropping below replacement rate increases our odds of survival as a species, as we can avoid the potential Malthusian catastrophe.
Our current falling birthrate has to be socially constructed. Having children used to an economic advantage, but this is no longer the case. Anything that changes that equation can reverse the trend, be it a social safety net, more technology, cheaper education, or more housing availability.
My basic point is that, insofar as the more liberalized and secular cultures of the present developed world can't sustainably maintain their current populations, there's a limited amount of runway for these cultures to persist. Eventually, we will either run out of immigrants and turn the entire world into one with a fertility rate below replacement, or some other, more fertile culture will out-reproduce or convert us faster than we can convert them, and that will be the culture of the future.
Perhaps it's possible that improvements in economic circumstances will allow fertility to rebound in secular cultures, but they've had the opposite effect so far. It's also worth pointing out that, by necessity, we're going to need some lead time to get the fertility rate back up. A fertility rate of 1.4 means that each generation is 70% the size of their parents' generation, and half the size of their grandparents' generation, and we're already having a hard time affording pensions as it is.
China, in particular, may have a massive self-inflicted demographic crisis on their hands due to the one-child policy. If Chinese parents during the one-child era had a strong preference for girls, maybe it would have worked out, but as it happened, they had a strong preference for boys. They might be able to handle the problem of caring for larger elder generations with smaller younger generations by offsetting the population decline with strong per capita economic growth, but the demographic issues involved with having large numbers of excess men will persist.
I'm also curious about the math that leads you to the two billion number. A Kardashev I civilization on Earth could sustainably support much more than that, and a civilization that advanced beyond Kardashev I, even more. By the time we run into the long term sustainability issues of our current population, or even of a population of 9-12 billion, it's entirely likely that we will be at such a level of technology that we can solve those issues anyway.
If there's an "obviously" here, it's that this is the direction in which current trends are heading.
The peasant class can reproduce all it wants. Managed Malthusian resource limitations will keep it from being a political or - eventually - even an ecological threat.
The predominant culture of the developed world is catastrophically infertile. The more progressive parts of these cultures are often the least fertile, while the more conservative and traditional parts are more fertile. But the offspring of these traditional subcultures often convert into the progressive culture, at least where that culture is more prosperous and holds higher perceived status.
Part of what makes religions so persistent is their evolutionary fitness from a memetic perspective. “Tell everyone you know to worship $DEITY, have lots of babies, and also kill/shun anyone who tries to stop worshipping $DEITY” is a great algorithm for maximizing the number of $DEITY-worshippers. “Never have sex or try to convert anyone” is a shitty algorithm, which is why the Shakers are practically extinct while Islam is on course to become the world’s largest religion (and may already be, if you subtract the plurality of western Christians who are behaviorally secular anyway).
I suspect some memetic program that survives contact with science, prosperity, and progressivism without turning infertile, if it ever evolves, will be the foundation of the future of human culture.