A lot of the reasons why scaling is necessary come down to reducing marginal costs and maximising profitability. Those are different equations to sustainability.
For example, a big part of an industrial farm's "efficiency" is down to reduction of labour costs and optimisation of logistics, but the actual environmental resource usage does not scale along the same curve.
Many of that "efficiency" is achieved by externalizing costs.
The farm can (must?) produce cheaper (or have bigger margins), by polluting both its environment and the very resources it needs to run on in the long term. It "externalizes" costs to the community around it and to a future.
Pollution, depletion, animal abuse, reduced biodiversity, sped up resistance to antibiotics, etc etc.
This isn't by any means "sustainable" in the literal sense: that it can continue, let alone grow, like this. We're on borrowed time already. It's very clear that the current model also cannot sustain billions. So dismissing alternatives because they cannot sustain billions is a poor argument.
For example, a big part of an industrial farm's "efficiency" is down to reduction of labour costs and optimisation of logistics, but the actual environmental resource usage does not scale along the same curve.