While we have a much more healthy variety and abundance of 'healthy foods' in our current epoch, we also have an abundance of carcinogenic and mutagenetic chemicals in our environment which are man-made and not a part of the earlier epochs. Supplementation can be benenficial in dealing with these dangerous chemicals.
I usually avoid using qualifiers. Not because I’m trying to hide anything, but because once I go down that path, I can end up writing a VERY long post and still not succeed in explaining every single qualifier. And after all that, my original point has not changed.
And you are correct, a “balanced diet” is an extremely broad entity. In considering any particular nutrient, it seems that there is a minimum quantity we must have, and a maximum quantity beyond which the effects become deleterious; anything between those two limits is fine. We’re very adaptable; that very adaptability means that the variability in what might be called a “balanced diet” is huge.
For the most part, I don’t worry much about carcinogens or mutagens in the environment. With all of the clean air and water regulations we have, those aren’t really an issue. Now, if we lived near a Mexican maquiladora (or however that’s spelled), where no attempts at all are made to control effluents, I’d be worried. In any case, much of our food, especially of plant origin, contains plenty of native toxins (many of which are potential carcinogens). We’ve evolved a very complex system to deal with them.