What is wrong with having a well balanced diet? Some days eat lots of fats, some days more carbs, etc. Extrema are not good for your health.
In fact, it's even sillier than that since extreme bursts of energy (which most of us call exercise) are demonstrably very good for your health.
If by "extrema" you meant "excessive things", then your point is tautological.
What are the food groups? What's the right proportion? Does it vary with age/ethnicity/nurture?
These are precisely the questions being examined, so telling people to eat a balanced diet without telling them what that actually is, doesn't help.
How are the food groups defined? Why isn't ethanol included as a food group? Do you consider fiber in its own group?
Taking "balanced" portions from a set of groups makes the context in which the groups are defined very important.