Then the socio-economic dynamics should be changed because they’re not any more sustainable than a constantly expanding population.

Your argument is flawed. Whatever reason people decide to not have children as individual couples or how they choose to express it is irrelevant.

There are limits to growth.

The consequences of growing beyond carrying capacity are what you need to be concerned with. Not a false narrative of people being too anti-social or selfish to not want to have children.

Choosing to not have children is part of a systemic choice to deliberately draw down a population in the least disruptive manner.

Overshoot leads to an uncontrollable and catastrophic population crash.

Make no mistake. We either choose the first one or the second one gets chosen for us. Only the first option secures any semblance of our civilization.