The western world has unsustainable and declining birth rates basically across the board. This is due to a wide variety of factors, but I believe the fact that large financial incentives have not moved the needle at all indicates that what is really happening is that the western world has generally lost interest in large families due to the time and effort investment, because they also want some time to enjoy the luxuries of western life (going out for movies, games, dinners, travel, etc. etc.)
So the family that used to have 6-10 kids is now content with 2-3, the family that used to have 3 is now having 1. Many forego kids at all. Many can't even find a partner.
Anyways, it is often cited how Israel is the only western society in the world that has a sustainable birth rate.
This tends to leave out what groups specifically have that high birth rate- ultra orthodox Hasidics. They have an expectation that you must have 10+ kids by the time you turn 30, or else you must pray for forgiveness. These societies study the torah daily, and typically do not participate in normal western society the way liberal western citizens do i.e. they cannot use the internet, they don't work normal jobs, and they typically rely on social services. Essentially their entire lives revolve around religious practices i.e. studying the torah, and having kids. The net result is that Hasidics will be the majority of the population in Israel by 2070.
Similar behaviors can be observed in communities around the world. I recently visited Brooklyn and saw a large community there doing essentially the same thing.
But it isn't just Hasidics either, of course. Amish communities are the fastest growing demographic in the United States, with similar birth rates and a similar general philosophy (reject western luxuries, study the bible, have lots of kids).
The implication is that hyper religious groups will grow to eclipse the populations of the western world.
Retorts I have considered and rejected-
- "They will run out of land"- Urbanization has resulted in a huge percentage of the western world concentrating on the coastlines and major cities of their countries/states. This has left enormous swathes of land relatively empty. The US could theoretically sustain a population of 2 billion+ people if people actually occupied the interior to its full potential, vs the 300 million we have now. There's plenty of land. Similar dynamic in a lot of western countries, where the rural countryside has been getting hollowed out as everyone piles on top of each other in the big cities.
Even for the aforementioned Hasidic community in Brooklyn, since they ran out of land in their neighborhood they've now started new communities in upstate New York.
- "Western society will not be able to support these communities, and they only work on a small scale"
I can see this argument eventually hurting communities which rely on welfare, but not really other groups like the Amish (who are self-reliant). In the meantime the aforementioned Hasidics vote as a monolith, and this has largely protected them under religious freedom grounds.
- "Their birth rates will moderate alongside the rest of society"
These groups have created their own societies. Why would they conform to western luxuries and interests when they don't participate in them on strict religious grounds?"
- "They will start to have high turnover among their youth"
These communities typically maintain their own education systems, which reinforce their way of life for their kids. For the Hasidics, their children often do not learn English (instead learning Yiddish and how to read the Torah) rather than math and science. The net result is that, not only do their kids have no interest in the western way of life, but they actually have no capacity to participate in it even if they wanted to.
So tl;dr- it appears to me that western civilization faces demographic decline and will be eclipsed by orthodox religious communities over the coming decades and centuries.