They were not designed to survive 7-8 decades regardless of diet, because by this time reproductive success (and therefore fitness and pressure) is zero for women and almost zero for many men.
Devil's advocate against this point: Grandparents might increase the reproductive success of their children's children, helping their genes propagate in accordance to Kin theory.
Devil's advocate against Devil's advocate of this point: Grandparents might also decrease the reproductive success of their offspring, since they must care for their own children as well as their parents. Hence, the near inescapable phenomena of dementia as age increases into the 100s, and the trait of Senicide (ex. Ättestupa) in many cultures.
My point being, maybe it is best to avoid categorical statements about "evolutionary perspectives."
edit: Correction, Ättestupa is mythological and not a good example of Senecide. The wiki for senecide has many other examples, however.