Much of the value system of the liberal west and the westernized world (i.e. what we call the modern world these days) is, if not explicitly anti-natalist, implicitly anti-natal. That is it values other things, like personal achievement and self-actualization over having children and all that entails (e.g. self sacrifice). This is the value system I grew up in, and I see the result. My dad had 35 first cousins, and the seven total cousins (including my siblings and me), have produced only seven offspring.
I really think we're in an interesting situation with current falling fertility worldwide and the prospect of declining world population. In a world with a shrinking, graying population, tight-knit subcultures with pro-natal value systems will be in a very strong position. They will grow rapidly as a share of population.
Furthermore, they will be youthful and vibrant when the rest of the world is old and getting older, which should afford better protection against youth attrition than they have today. In 2124, a young person leaving their, Mormon, or Traditionalist Catholic, or Ultra-orthodox Jewish community for the secular world may feel less like a small town kid moving to the city and more like a kid leaving the city to live in a an economically depressed small town.
In democratic societies these communities will also have growing political power. We see this today in Israel, where secular women's total fertility rate is 1.9, while Haredi (ultra-orthodox) women's TFR is around 6. The Haredim was a microscopic minority at the founding of Israel and now they are a voting bloc big enough to topple governments.