It happened in the real world, ergo the issue lies with your understanding of the system we're talking about and not with its inability to conform to your model.
You assert these things very confidently, but without any evidence. How exactly do we know that this state of affairs existed in human prehistory?
You say:
You don't need to invoke group selection in the form you're thinking of -- the cultural "mutations" you're positing can't gain a foothold until some branch of humanity has access to a lifestyle that makes it advantageous to defect like that. Forager societies don't have that incentive because if they overtax their resource base here and now they have to move, and for most of human prehistory (and the modern history of hunter-gatherers) the population densities were low enough that this gave the affected area time to recover, so when someone came back, things were fine again.
This, however, provides no answer to the question why individuals and small groups wouldn't defect, regardless of the subsequent collective consequences of such defection. You deny that you postulate group selection, but you keep talking in a very strong language of group selection. Earlier you asserted that "population growth unto itself is not a goal or a value of forager societies," and now you say that "[f]orager societies don't have that incentive." How can a society, i.e. a group, have "values" and "incentives," if you're not talking about group selection? And if you are, then you need to answer the standard objection to arguments from group selection, i.e. how such group "incentives" can stand against individual defection.
I have no problem with group selection in principle -- if you think you have a valid group-selectionist argument that invalidates my objections, I'd be extremely curious to hear it. But you keep contradicting yourself when you deny that you're making such an argument while at the same time making strong and explicit group-selectionist assertions.
You assert these things very confidently, but without any evidence. How exactly do we know that this state of
affairs existed in human prehistory?
Archaeological evidence regarding the health and population density of human beings and their dietary habits. Inference from surviving examples. The null hypothesis, that we didn't start with agriculture and therefore must have been hunter-gatherers for most of our existence as a species. The observatiion that the traits generally associated with the Malthusian trap are common experiences of agricultural societ...
From Mike Darwn's Chronopause, an essay titled "Would You Like Another Plate of This?", discussing people's attitudes to life:
Conclusion, graphs, and references in article. As usual, I recommend reading Chronopause.com as Darwin has many good articles; to quickly link a few: