Re: Living on..
As someone who was made to sit through the program where a family live in the 50's for a week, eating only 50's food and having only 50's appliances, etc. I think we could easily extrapolate back to a time when this could have been true of every human on the planet. But we're still here.
Rickets is pretty easy to cure. Sunshine and a drop of milk occasionally. The problem was only ever rife when indoors-living became the norm, or - like scurvy - if you were on a long sea-voyage away from fresh-food.
The thing about nutrition is that what's NEEDED it pretty miniscule, in tiny quantities, and naturally present in an awful lot of foods anyway. Malnutrition causing things like rickets is rare except where it's COMPLETE malnutrition - where no thought is given to food variety and people are given one particular food, or no food at all, for long periods. Just in the article, egg has enough vitamin D in it. A splash of milk (fortified in some countries!) provides enough calcium, etc.
The body is surprisingly durable. You would have to have a very, very prolonged and static diet in order to induce malnutrition in someone who has a free choice of modern food, no matter the spending limit.