[simonh]

I share your doubt. As for sustainability, the key to that is efficient utilisation of resources. I don’t see how that can be possible without scale efficiencies.

Basically there are trade offs between population level, standard of living and sustainability. More of any one of those will require us to make do with less of one or both of the others. A more efficient economy can give us more to distribute between them.

That’s broad strokes. There are cases where some efficiencies compete with some standard of living concerns for example. It would be a poor world in which there were no artisanal crafts.