Except its not nonsense - doctors deal with people with this type of narrow diet all the time. Or, not sufficiently varied diet on a day to day basis.
Humans are also awful statisticians - fat people don't think they eat much. Thin people think they eat tons of food. Nobody accurately assesses what they do and don't eat, and our entire society is negatively geared towards maintaining healthy eating patterns anyway (your above example of variety will get blown to hell for most people past a few stressful weeks).
Yes, technically this is unnecessary but that's hardly the point.
Humans are also awful statisticians - fat people don't think they eat much. Thin people think they eat tons of food. Nobody accurately assesses what they do and don't eat, and our entire society is negatively geared towards maintaining healthy eating patterns anyway (your above example of variety will get blown to hell for most people past a few stressful weeks).
Yes, technically this is unnecessary but that's hardly the point.