I've read about other people doing this in the past. There was a study by a guy back in 1912, Dr. Mikkel Hindhede who wanted to prove that a very simple diet could keep a person perfectly healthy.
One of his laboratory assistants, Frederik Madsen, volunteered for the experiment. For an entire year, Madsen ate nothing but potatoes and milk (sometimes described as potatoes + buttermilk or potatoes + a little fat). He stayed totally healthy the whole time, with no deficiencies, full energy, and perfect physical condition.
Also, I have personal knowledge of this style of eating. Because I do almost same thing. Six days a week, I only eat potatoes and milk. On saturdays I eat beans, rice, and corn tortillas. (I was inspired by the Hindhede study, by the way)
I've done it for almost 2 years and no issues at all. I did it mostly for savings, since I wanted to cut my budget really low, so I could hurry up and pay off my house (which I had already been working on paying off by working overtime and side jobs.)
It worked. I paid off my house last year. Now no mortgage at all. :)
And I work out every day, and last medical check up showed that I'm ideal weight, ideal health, no issues at all.
After reading how many Lemmy posters are overweight, um, I think a lot of people should try the potatoes only thing. It sucks for first 2 weeks, but then your body adjusts tho.