And do we really have to change our diet as we get older? Besides being reared by their mother in the earliest part of their lives, do other mammals change their diet as they get older?
https://ojs.ethnobiology.org/index.php/ebl/article/view/721/...
Also potatoes are almost complete if you get sunshine for your vitamin D, but you'd be deficient in at least B12. I think most wild animals eat multiple things so it's not really fair to try and find a single food that fits.
On changing diet, your body becomes more sensitive to deficiencies. If you're lacking in calcium, for example, and you break a bone, it's a lot more serious at 75 than it is at 25. The NIH recommends a slightly higher calcium intake in young people (for development) and over 50s (for preventative maintenance). Maintaining a young person's RDA your whole life wouldn't do you any harm though.
https://www.bones.nih.gov/health-info/bone/bone-health/nutri...
In the animal kingdom there's the concept of food chains/trophic levels. This is why ecosystems are fairly fragile if a provider species is removed, e.g. by extinction. Humans have learned to adapt to wildly different resources, and we have thousands of years of knowledge and culture about how to prepare food in a way that's nutritionally beneficial. There are several "staple" human foods which we've learned to eat, like cassava, which contains cyanide when raw.
Because we live long enough (medicine, food abundance, proper shelters, &c.) that optimizing food intake matters if you want to get the best out of your body.
You can probably eat bigmacs all your life and make it to 60+, but you'll be far from your potential.
The fact that we're mammals doesn't mean we have to live like chimpanzees, but no one is stopping from living like one ("do we really have" -> no we don't)