It is pretty easy to cherry pick a paper advocating whatever you want to advocate for vitamin D. The paper you cite is entirely founded on another paper about fixing the daily estimate. That paper states about its improved estimate: "As this dose is far beyond the range of studied doses, caution is warranted when interpreting this estimate."
I have no doubt that the mainstream NIH advice is not optimal. The best thing for everyone to do though is not to recommend high dosages for everyone but instead to recommend getting vitamin D levels tested.
I noted the levels needed to maximize intestinal calcium absorption because this is the best studied aspect of Vitamin D. Rickets was the main reason Vitamin D was discovered and supplemented.
Vitamin D level blood testing is imprecise - I would not consider any one test of Vitamin D levels to be accurate.
This person's cited paper corresponds with the calcium absorption data that suggests around 8,000 IU D3 is where calcium absorption begins to plateau and more vitamin D leads to minimal additional calcium absorption.
Since I haven't been going outside, I take vitamin D 10,000 IU daily. You are correct in that one shouldn't just take the advice of random internet strangers - I should be more careful in posting. My personal experience with Vitamin D has been extremely positive and so I get excited about sharing what I know - of course I had to work and couldn't actively respond to this thread as much as I would like.
Its nice to see someone not get upset about the criticism!
Things that work well for one person can be dangerous for others. It's fine to share your positive experiences, but it's another step to be recommending the same to others.
There are a lot of other factors besides calcium absorption to consider. In fact maximizing calcium absorption with vitamin D likely has negative consequences unless there is proper vitamin K intake as well.
I have no doubt that the mainstream NIH advice is not optimal. The best thing for everyone to do though is not to recommend high dosages for everyone but instead to recommend getting vitamin D levels tested.