That’s true. There might be all kinds of nutrients we don’t know about. But if we’re basing our diets on nutritional needs that we have no evidence for, then there’s no reason to suppose any one diet is better than another. A diverse diet, whole foods could be deficient too, if we don’t know what those optimal micronutrients are.
There’s plenty of history that supports the viability of simple diets. And as I said above, there are people who live decades on monodiets such as just meat. And of course, there are the millions of people living here in the United States that don’t bother to eat much but boxed foods from the supermarket and fast food joints. If there were nutritional deficiencies, I imagine we would have noticed.