TBH I think that it's easier to have a balanced diet now than ever before.
In years gone by people ate food that was in season and in climates with a very cold winter spend most of the winter subsisting on heavily salted or picked produce. The choice of fruits was pretty much limited to what was grown nearby and in many cases to what they could grow themselves.
Until the industrial revolution even the wealthy were only one bad season from starvation and generally had attrocious diets.
I've got a photo somewhere of the avaerage 50's family standing in front of the food they consumed in a year. There was not a great deal of variety and there was a lot of fatty and heavily salted food.

TBH I think that we're incredibly lucky in that we have access to such a variety of healthy and nutritious foods. If I was living on US$5 a week I might find it hard to have a nutritious diet but since I don't I guess I think my glass is pretty much full.

IMO the statement that it is impossible to get a balanced diet is taking a totally ahistorical view of nutrition. It's never been possible yet the human race has thrived and prospered despite it.