> And most importantly of all - this isn't an option. Societies that fail to maintain themselves will simply die off and end up being replaced. Much of what I'm saying here runs face first into contemporary ideological ideals, but the reality of what I'm saying will win simply because contemporary ideals are not self sustaining.
You are right to an extent.
First, if the inclination to have more or fewer children is at all hereditary, there is a strong natural selection effect over time. Evolution in action.
Second, (sub-) societies don't have maintain themselves via their _own_ children. Priests in the Catholic church were famously barred from having children for at least a few hundred years by now. Yet, the Catholic church persists. Similarly, throughout most of history cities had below replacement level fertility, just because we didn't have the hygiene and medicine necessary to keep the diseases at bay. Yet, cities persisted.
You can say that they have been 'replaced', but so are families every generation. Drawing a strict line is only possible, if you put an undue emphasis on genes only.
You are right however, that for a culture or 'ideology' to persist, you need to replenish the pool of people in some way, either with children or converts/immigrants. (Or, I guess, you can figure out immortality for your members?)
Almost by definition, migration/conversion can only be an option for the most appealing of societies: those migrants have to come from somewhere; they are other people's kids.
The Catholic Church persists because of followers of the religion, not Priests. That those followers have healthy fertility rates is the main reason it, and quite a number of other religions, are growing to rapidly growing - in spite of the perception many in the West would have about the global increase of secularism. In fact we're likely to become a far more religious species in the future thanks to secular individuals removing themselves from the gene pool with a disproportionately high frequency.
And replacement is not about genes, but about culture. Europe experimented with mass migration with peoples of very different cultures. The idea is that they would nonetheless be assimilated, integrated into the culture, and everybody would grow all the richer for it. It instead just led to the emergence of countries within countries, increasing violence, extreme social divides and extremism, and is arguably playing a significant role in the ongoing decline of Europe. Migration is, in general, a good thing - but if it ever goes beyond a rather small scale, you're just setting the stage for your own downfall. Migration was even one of the major causes of the fall of the Roman Empire!
> The Catholic Church persists because of followers of the religion, not Priests. That those followers have healthy fertility rates is the main reason it, and quite a number of other religions, are growing to rapidly growing [...]
The same logic can apply to western culture. People already living in the west are the 'priests'.
What you're arguing here is largely ahistorical. Immigration to the US was relatively small scale and almost entirely from highly compatible cultures. There's a decent write up here [1]. Some datums: Immigrants today (including illegal) are about 14.3% of the population - that's about the same level as it was following the great migration of the late 19th century shortly before it started to become an issue leading to immigration being clamped down on hard, reaching a low of 4.7% in 1970. [1]
Fertility, when shared amongst a population, has an easy to model effect on population. It's a population scalar of fertility_rate/2 per ~20 years. So a fertility rate of 1 would mean your population is dropping by 50% every 20 years. That is compounding/exponential and never ends until you go extinct or start having more babies. Compensating for this by immigration is basically impossible, and at that scale you will quickly become the country of origin of immigrants in any case. Of course we're not even especially close to a fertility rate of 1 yet, but that is the trendline and really the same argument even applies to rather higher levels of fertility.
You are right to an extent.
First, if the inclination to have more or fewer children is at all hereditary, there is a strong natural selection effect over time. Evolution in action.
Second, (sub-) societies don't have maintain themselves via their _own_ children. Priests in the Catholic church were famously barred from having children for at least a few hundred years by now. Yet, the Catholic church persists. Similarly, throughout most of history cities had below replacement level fertility, just because we didn't have the hygiene and medicine necessary to keep the diseases at bay. Yet, cities persisted.
You can say that they have been 'replaced', but so are families every generation. Drawing a strict line is only possible, if you put an undue emphasis on genes only.
You are right however, that for a culture or 'ideology' to persist, you need to replenish the pool of people in some way, either with children or converts/immigrants. (Or, I guess, you can figure out immortality for your members?)
Almost by definition, migration/conversion can only be an option for the most appealing of societies: those migrants have to come from somewhere; they are other people's kids.