Fair enough, but I still don't think we can show that humans have evolved to the point of achieving optimal health on a high carb diet in a few thousand years.

Fair enough, but I still don't think we can show that humans have evolved to the point of achieving optimal health on a high carb diet in a few thousand years. Grain eaters are still overall pretty healthy, so they can still accomplish the beneficial things you describe, allowing their less-than-optimal genes to carry on.