Where does the idea that potatoes are "almost-complete" in terms of nutrition come from? If you look up nutrition facts for potatoes:

-1 potato contains 110 calories

-Lets say our hypothetical adult will eat 10 potatoes to hit 2200 calories

-The only nutrients which potatoes provide more than 10% recommended daily value are Iron, vitamins C and B6. The iron is not heme iron and therefore may actually be worth less, but we'll say it's enough just for the sake of argument. Potassium and phosphorous are close at 9% and 8%, but everything else is lower.

-The fat content is negligible, which is a problem because your brain is made out of fat

-20 grams of incomplete protein means your body can't repair muscle or bone adequately.

-No cholesterol, which also has negative outcomes

This is a high calorie starvation diet.