Urbanisation has been a self destructive symptom of sophistication in society since the dawn of history.
Humans (for whatever reason) over several generations do not breed at full replacement rate in larger urban areas.
Cities only grow and perpetuate due to continual rural-urban migration. In recent decades particularly perpetuated by tertiary education.
Don’t believe it?
The general trend in a country/civilization is that there are social, climate and agricultural circumstances that lead to a ‘green revolution’ where subsistence farming flourishes as does the land dwelling farming families. This usually involves food commodities that can be stored and transported. As the general health and fertility improves there is a ‘baby boom’ which leads to surplus labour above the requirements of subsistence farming. At this point you have the formation of standing armies and division of labour, leading to ‘peace’ (Pax Romana style).
Rapidly, looking back through the lens of history, urban civilisations pop up overnight like mushrooms. There is for a while (6 or so generations) a positive feedback loop where urbanization flourishes and the resulting industrialization improves rural life.
Look to all the great historic civilisations how they have waxed and waned.
For example: Egypt, Persia, Greece, Roman, Mayan, Aztec
BUT it must be emphasized throughout this growth stage that Urban Growth is largely the result of Rural-Urban migration.
Eventually the rural populations become aged and depleted as they have sent too much of their ‘breeding stock’ to the cities.
"In our time all Greece was visited by a dearth of children and a general decay of population," lamented the Greek historian Polybius around 140 B.C., just as Greece was giving in to Roman domination. "This evil grew upon us rapidly, and without attracting attention, by our men becoming perverted to a passion for show and money and the pleasures of an idle life."
The demographic dividend window closes, without fresh blood the cities become sickly and plagues pass through. There are not enough healthy young men for an army strong enough to keep out those who covet the accumulated wealth. There are Malthusian limitations on the food and transport.
Some ancient civilizations like Rome pushed things to the limits with a massive empire channeling resources and man power (slaves) into to urban areas.
The industrial revolution has changed the dynamics with fossil energy rather than the energy of living flesh.
In our modern society Japan is the pathfinder on the journey of demographic transition, they reached that point earlier than most countries due to no post WWII baby boom (they legalized abortion in 1948) and have very limited immigration. Their economy continues by vitue of automation and high value exports.
New Zealand has been hovering at the point Japan reached in 1990, since 2015 the inevitable delayed by (proportional to population) very high immigration and again high value exports, including international tourism.
To stem or even reverse rural depletion it is important that rural employment opportunities are developed and preserved.
A further change has now happened with the ‘information age’ allowing as much connectivity in the rural setting as in the city.
We are just waiting for the autonomous transport revolution to negate remoteness as the final stage of the “4th Industrial Revolution” https://en.wikipedia.org/wiki/Technological_revolution
Autonomous transport will facilitate a distributed (as opposed to centralized monolithic) socioeconomic society where economy of scale has little value.
It can be the return of ‘small is beautiful’ and breathe new life in to rural society.
Transportable, self powered distributed biocoal plants (scaled to suit logistical limitations) that can be taken into the forest are the only feasible way of using woody biomass.
Development of efficient distributed electrical power generation from biocoal at various access points on the grid is the next stage to completing the link between planting 1.5billion extra trees and providing extra electrical power to fulfill the need created by the electrification of Process Heat and Transport.