Just because those genes exist doesn't mean they're evenly distributed. For example, look at the spectrum of lactose tolerance: some people do just fine, some people get a little gassy, some people get severely ill.
Yes but that mostly is distributed according to dairy consumption from around 500 years ago. Grain consumption is widespread across the globe in almost every culture.
10K years hasn't been enough for cats, apparently. Plus, the lifespan of a dog is ~7 times shorter than the lifespan of a man, which will give them at least 7 generations more to adapt.
Obviously there has been some adaptation to the change of diet, and it is also obvious that this diet was not letal to human (or nobody would have adopt it), but this does not implies that it is optimal for human consumption. Most people in my region are not lactose intolerant... but do you know what? I threw up every time that my mother had me drink a glass of milk because it was "healthy" and good for my bones. If adaptation depends that much between humans (the % of lactose intolerant or lactase persistent is not the same in the US than in China or Africa, for example), trying to correlate anything with animals seem completely off to me.
More like 2-3 times shorter. Humans have barely gone past 30 years of age for most of the last 10k years, and I don't see how dogs' lifespans were improved in recent centuries.
Humans lifespan was that low since the high mortality rate the first months of life. After the first years, the expected lifespan increased dramatically. A humans need a few years to be able to reproduce and those who were able to have a longer lifespan. Even if the dogs didn't improve their lifespans (which honestly, I doubt they didn't) we are still in a similar ratio.
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.
In fact, 10k years is enough for dogs to have evolved to digest carbs a lot better. There's even specific genes that are known to help that.