Economic success is associated with reduced fertility, not greater fertility. (And it's almost certainly causal.) If regions don't have economic miracles, that will mean higher population, not lower.
One part of this came up at the end of the article, namely that increased childhood survival rate leads to decreasing number of births. It makes a lot of sense to me that this would correlate with overall improvements of living standards.
If you want to have a family four children, I'd think it's pretty universally agreed that it's better if you only have to give birth four times to do it than five, because all four babies survive childhood.
That doesn't negate that there are other factors, especially in countries that already have low birth rates and high childhood survival rates. It was just a factor I hadn't paid much attention to before or heard people mentioning very often.
In theory it would make sense that fertility falls a bit with lower childhood mortality, but your explanation doesn't explain the data that lower childhood mortality is associated with fewer surviving children.
Not an expert but AFAIK these demographic transitions are when an economy has the highest share of its population as economically active adults. It's a well known challenge to use that opportunity to develop and become rich enough to deal with the following ageing population and avoid the middle income trap.
If we can continue to figure out basic needs better like housing and healthcare then it may not always be the case that one must sacrifice progeny for economic progress.
Hell just high speed rail could encourage populating less desirable areas if a 30 minute trip to the beach was a weekend event.
This is what I've always heard, but I've also heard that the financial precarity of young people in the west is leading them to have fewer children. Which seems mildly contradictory.
If you are precarious now, but expect or hope to be better off in future, you will delay having children. If you don't expect better prospects, then children are the best available investment in your future.
If you give people resources conditional on them having children (e.g., free child care), they will of course on the margin have slightly more children. But it's a mistake to interpret this as "financial precarity" causing reduced fertility, because if you give them more unconditional money they have fewer children.
