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> if the society consumes a majority of its productive members without insuring their reproduction, then they'll be selected against.

This assumes both that productivity is heritable and that the nature of productivity is fixed. The former seems contingently true (assuming the latter), but the latter seems false, as the forms of work considered "productive" tend to shift over time.

The other Problem is the implicit assumption that "[economic] productivity is good", which is an entirely different class of argument than above.