I find the grandparents hypothesis compelling at first glance but it sort of begs the question, why don't we live indefinitely in the first place? There are obvious answers, we make tradeoffs to improve performance early in life at the expense of long term function, but it doesn't seem like the reproductive benefits of caring for a grandchild that only shares a 1/4 of your DNA necessarily tips the scales of selection towards longevity. Especially when, in theory, men remain fertile their entire lives and thus there should have always been some selection for longer life spans. You would expect the reproductive benefits of a 70 year old caring for their own child might be at least comparable to 70 year old caring for a grandchild.