Population members aren't useless to preserving genes once they're done having babies.
Elders use and share collected knowledge (particularly skills that take a long time to master), care for babies while the parents are hunting/etc., help with gathering/cooking/cleaning/etc.. If they are all sick & dying instead of doing this work, your group isn't going to do as well as the competing group whose elders are in good shape.
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.
Population members aren't useless to preserving genes once they're done having babies.
Elders use and share collected knowledge (particularly skills that take a long time to master), care for babies while the parents are hunting/etc., help with gathering/cooking/cleaning/etc.. If they are all sick & dying instead of doing this work, your group isn't going to do as well as the competing group whose elders are in good shape.