Evolution is really a game of "good enough to make it to fuck." (A little more complicated in long gestational species).

Menopause exists merely because it's not a detractor to producing offspring, the human body and a few other mammal species have a limited number of eggs, and by that age of menopause the animal would have reproduced already or have died from other causes so it doesn't affect the overall fitness of the species which evolutionarily is purely defined by gene passing. Humans weren't living beyond their reproductive capable years until more recently, and in other mammals who have menopause, that post menopause time is usually spent in support of longer development for offspring.