Yeah. It seems like this discussion rests on the assumption that in a future, primitivist society, people will meekly accept the occasional famine because it's better for the earth. I think that people in the midst of such a famine would be desperate to do anything they can to save their children, their friends and family, and themselves. Including obtaining food by whatever means necessary, earth-friendly or not. Including attacking the neighboring tribe who is rumored to have more food.
It's been pointed out earlier, but actual hunter-gatherer societies don't necessarily have such sustainable lifestyles anyway. They just lived in such small numbers in the past, that they were only able to destroy small areas. And the chance of a small band on the edge of survival being peaceful is low -- maybe if they life on a remote island where there just isn't the opportunity to go to war with others.
I think there's also a disconnect between this imagined mass-human-extinction event, and the idea of everyone peacefully adopting primitivism afterwards. I think after a terrifying event that kills off the vast majority of the human population, the survivors will be jumpy, paranoid, & desperate. Not to mention traumatized by their losses, and determined to save those friends and family who remain to them at all costs. And if they're always facing the possibility of death in the short term, they won't have the luxury to consider the long-term consequences of their actions. To me, that sounds like a recipe for increased xenophobia and constant warfare, since caring about "other" groups is a luxury for those who feel secure from attack. And I think such a population would be much more likely to adopt unsustainable-but-good-at-the-moment methods of obtaining food, and would be in furious pursuit of regaining any technologic knowledge they can.
A more promising path for sustainability is increased technology, not decreased. And increased urbanization, where each individual person takes up a smaller share of resources.