I really like the general philosophy behind what you are saying. It has a nice ring to it. But saying to eat a well-rounded diet is not advice. Nobody has any good idea what a well rounded diet is.

You must eat fat (to obtain essential fatty acids). You must eat protein (to obtain essential amino acids). There are certain vitamins you must eat. And there are certain elements that you must consume.

The real question is how much of the required nutrients is enough and how much is too much?

What ratio of macronutrients (carbohydrates, protein, and fat) is ideal for maximizing health? Are there trade off for different ratios? For example, carbohydrates are not required for life, but most diets obtain a large portion of daily caloric intake from them. What percent of calories should be from carbohydrates?

Nobody yet knows what a well balanced diet is. In absence of that information, your advice is pretty good. Eat lots of different stuff.