Everything in moderation?

A response to my pleas for people to change their diet I frequently hear is, "Doc, I believe we should eat everything in moderation." I guess that means we should eat a wide variety of foods because we aren't really sure which are the good and which are the bad, so to make sure we aren't missing out on some vital nutrient just eat everything.  A balanced diet include only foods that are healthy. We know that concentrated carbs are not essential to a healthy diet. Their presence only unbalances.  You shouldn't include carbage in to a healthy diet and expect an improvement.  Are human's designed to eat everything in moderation?  Many species of animals eat a very narrow diet and do very well.
The Koala is a great example. It subsists almost entirely on eucalyptus leaves.
Tigers and other predators are perfectly healthy without eating any salad.
Giraffes, acacia leaves.
There are human examples of  grain free diets:
The Inuit (eskimos)  Lived for generations in excellent health subsisting on their traditional diet of mostly fat and protein. 
Maasai-tribe in africa- Eat a virtually exclusive animal based diet.

  

Those people ain't slim because they're starving. They're slim because they're metabolically healthy. 
Unfortunately as these isolated peoples transition to the global industrial food supply they prove susceptible to modern obesity and disease.