Like I said in the comment, this is
my take on reports, not the current scientific consensus.
About the desperately needed citations, let's start from these two:
https://pubmed.ncbi.nlm.nih.gov/17413106/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6180659/
The first is a study in South Indians. Even among rural workers spending long hours in the tropical sun, the study finds >80% of people had Vit D levels below the 'sufficiency' level of 30 ng/mL, in this otherwise healthy cohort. 44% of them had levels below 20 ng/mL. While this study doesn't report bone density, other studies on rural Indians suggest the osteoporosis prevalence at ~ 1% of the non-elderly population. There are no long term follow up studies in the rural indian population, but it's a reasonable assumption that individuals with very low Vit D levels (ie, < 12 ng/mL) were more likely to develop osteoporosis later in life. The study also reports low dietary calcium and high intake of dietary phytate, which are suspected to reduce circulating vit D levels. Do these necessarily lead to long term harm? It doesn't seem like it. Other reports suggest that lower bone density among rural Indians is most strongly linked to lower lean muscle mass. Greater the load bearing exercise, the better the bone density.
The second study is among healthy blood donors in Nairobi, with 95% of subjects with Vit D levels between 12.73–22.07. In both studies, the participants live in tropical environments exposed to high levels of sunshine.
By setting the optimal level of Vit D at 30 ng/mL, it pathologizes these otherwise healthy subjects. The concern is that the long term supplementation required to achieve 30 ng/mL would have unintended consequences, such as inappropriate calcification. As the Nairobi study points out, while low levels of Vit D are linked high levels of parathyroid hormone, beyond a certain point increasing vit D levels are linked to higher PTH levels. PTH leaches calcium out of the skeletal reserves and raises calcium levels in the blood. The deleterious effects of Vit D hypervitaminosis are due to high circulating levels of calcium. The Nairobi study shows that the lowest PTH levels were associated with vit D levels ~ 30 ng/mL, while individuals with vit D of 50 ng/mL had PTH levels similar to those with Vit D levels around 15 ng/mL. From the figure in the paper, it looks like if the goal is to minimise PTH, vit D levels < 15 ng/mL are sufficient.
Which brings me to my initial assertion: brown and black people with plenty of sun exposure have naturally lower levels of circulating Vit D, with the average around 20 ng/mL. The Nairobi study points out that greater sun exposure did not significantly increase Vit D levels in this group. They do not seem to suffer greatly elevated levels of bone abnormalities due to this, or any other obvious health concerns. To me, this is suggestive that these levels are naturally equilibrated to be in the 20 ng/mL range, and attempts to push them higher might be deleterious. Setting the standard at 30 ng/mL unnecessarily pathologizes these otherwise healthy people, encouraging supplementation, which is not without health risks.
Dark skinned people who live in high latitudes probably need greater supplementation than their light skinned neighbours, which is reasonable.