What question is really being asked - did we evolve the ability to digest carbs? We've apparently always been able to do that. Or did we evolve the ability to eat large amounts of carbs without risking various ills like type II diabetes and heart disease?
The paleo hypothesis is that we haven't adapted to carbs to the point that they don't cause disease. Carbs aren't dangerous enough to kill us before we reproduce, so why would we expect the genes to be selected out?
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.
The paleo hypothesis is that we haven't adapted to carbs to the point that they don't cause disease. Carbs aren't dangerous enough to kill us before we reproduce, so why would we expect the genes to be selected out?