You're right it's not a good argument. A better argument would be taking a look at how long humans have been afflicted by things like heart disease, diabetes, and different types of cancer. I'll give you a hint, they're relatively recent developments in the history of our species.
RE: Dying by 40, how much of that data takes into account infant deaths and things like clean running water? I don't think early humans were dying from "artery clogging saturated fat" and if that was the case, wouldn't we have generally evolved not to eat things that killed us?
> wouldn't we have generally evolved not to eat things that killed us
I'm not convinced that's true, evolution only works to improve fertility; e.g. women after menopause have no evolutionary purpose. Even for men it's not clear why a longer life-span would result in increased reproductive success (maybe men have most/best children when they're young?).
Although I must admit it's surprising that there wasn't more evolutionary pressure to increase the "health-span" of people (i.e. women's fertile years, and men's and women's "peak physical performance" years) - maybe having too many children isn't a good thing (decreases genetic diversity pool and is thus bad for the whole species), or there was another kind of evolutionary trade-off (e.g. human females "waste" eggs by having a "concealed" ovulation every month, instead of being "in heat" (like in dogs) or by ovulating only after sex (like in cats); however, this allows for other evolutionarily beneficial adaptations [1]).
A long lifespan for men results in more reproductive success. Even one offspring from an old man is a success that the man who dies young doesn't have. This is a small factor but small can be enough.
There is also the factor that old people without children can help find food and defend the whole tribe thus ensuring better survival of their grandchildren.
"Atherosclerosis was common in four preindustrial populations including preagricultural hunter-gatherers. Although commonly assumed to be a modern disease, the presence of atherosclerosis in premodern human beings raises the possibility of a more basic predisposition to the disease.
This isn't a good argument, as our ancestors have also been dying by 40 years of age.