My doctor specifically prescribes 5000 IU as a maintenance dose and 15000 IU to people severely deficient. These are numbers he got from treating hundreds of deficient patients and testing his patients blood levels every 6 months. I imagine a lot less is needed if you are in the sun for a good portion of the day every day but for most of us who spend a decent amount of time inside it seems to be what’s necessary.
My experience correlates with your doctor's: I had to take more than 5000 IUs per day for about 3 months to reach the bottom of the normal range for 25-OH-vit D.
Did you read the open letter (original link of thread)? There is an entire paragraph and footnote with 2 refs talking about the problem with the current government guidelines.