I think part of the problem is that a 'balanced diet' is also pretty poor in micro nutrients/vitamins/etc because modern farming techniques mean the soil itself is pretty exhausted, with just the bare minimum being added back to yield the next crop.
I get the sentiment but this is very much an exaggeration. Vegetables might have 5% to 30% less micronutrients than 100 years ago if you believe in the accuracy of micronutrient measurements from 1940 (you shouldn't). But there's good evidence that the soil isn't to blame. Rather, we have switched to varietals which grow quicker and have less time to pull good stuff up from the ground.
In any case, vegetables are now many times cheaper and more accessible than before. As a species we have hundredfold better access to nutrition than a century ago.
The fact is most people feed themselves mostly on flour and oil. That's what's causing a malnutrition epidemic, not poorer vegetables.
