Sure, it is quite far-fetched. However it is extremely uncommon that we experience unified social trends all across the board, from liberal Finland or Japan to North Korea and Taliban-run Afghanistan. Usually there are odd reversals and exceptions here and there; not this time apparently. And we still lack a satisfying theory that could account for fertility decline in every country.
Decline happens also in territories that had been urbanized decades or even centuries ago but had positive fertility rate until 2010s. As I said you can pull a patchy blanket of micro-theories explaining each region but not one theory that accounts for them all.
Yes, but it happened in stages. The fertility fell each time people moved into higher density areas.
You can even see mild recovery when de-densification happens. It's very interesting to compare the fertility rate in Denmark and Netherlands:
https://www.macrotrends.net/datasets/global-metrics/countrie...
https://www.macrotrends.net/datasets/global-metrics/countrie...
You can see the dip and a recovery in Denmark and essentially no recovery in Netherlands (until post 2000, but that was due to immigration). Why?
Here's the answer:
https://www.macrotrends.net/datasets/global-metrics/cities/2...
https://www.macrotrends.net/global-metrics/cities/21930/amst...
Denmark de-densified its cities during the late 70-s (that's why Copenhagen is the world's most liveable city, btw).
We do. It's called "urbanization".
Large cities are inherently inimical to living in large families. And yes, it was apparently the case even in the Roman Empire.