To be fair, humans can continue to improve the success of their descendants long past that age, and presumably as a result of that selection pressure, we live far longer than any comparable mammal. Something that kills you when you’re 50 or 60 will still be selected against, to at least some degree. It’s just a question of how much it’s selected against, vs. how much benefit is gained from a trait at younger ages.
Without “maybe”, fiber is the non-metabolizable parts of food. Those complex carbs which cows can metabolize and we can’t? For us those are fiber, for a cow they’re not.
Even a cow can’t digest that fiber on its own and requires symbiotic microorganisms to do the actual fermentation.