Balance in diet seems to be the one principal that will never go away. I'm suspicious of any diet that eats too much of one thing, especially if it claims the other things are harmful.
The term "balance" is meaningless unless we know what kind of values are ascribed to "too much" which is exactly what we are trying to sort out. No one talks about eating a "balanced" amount of high fructose corn syrup, in terms of nutritional benefit.
I agree, it depends on the definition of the spectrum or range of values that would then provide a definition of balance. Maybe the food groups could be the level at which this is defined, so too much or too little of one food group is probably not good. A good example is the meat-only diet that became a recent fad. People can claim all kinds of benefits there but in the long run it's just not balanced, and we're not built like apex predators in nature.