Have you forgotten the prevalence of goiters prior to iodine supplementation?
And how about rickets prior to Vitamin D supplementation?
Vitamin D RDA’s were developed to stop rickets and rickets only. To that extent, it was short sighted. And Vitamin D deficiency, while it can be corrected with “pill popping,” is not a matter of eating a “balanced diet.” It is a matter of latitude and living indoors.
I see people with what I call sub-clinical malnutrition every day. You can be fat and malnourished.
Foods of animal origin are a good source of Vitamin D. So are a few minutes of sunshine every day.
All of the diseases of malnutrition stem from people not eating a balanced, varied diet, often because of their geographical location. Ricketts was caused by Vitamin D deficiency. Scurvy, caused by lack of Vitamin C, was a problem with sailors, who would spend months at sea without fresh produce. In many cases, the specific vitamin was discovered because of the disease its lack caused.
Iodine is a problem because it comes from seafood, and people who live inland may not have much access to seafood. Thus, iodine was introduced into salt to help prevent goiter. If you eat sea salt, it should have already have iodine in it.
For the majority of people, eating a balanced diet is sufficient to meet their vitamin and mineral needs. If you have a specific medical condition (as you indicate you do), then there may be need for specific supplements.