the Vitamin D cult is a bit insane

it is -exactly- like Linus Pauling and Vitamin C cult in the previous century

Vitamin D is very important

Too Much is as bad as Too Little

The "RDA" is too low at 600IU and should have been changed to 2000IU decades ago

It can help prevent certain diseases and illnesses

It CANNOT cure any known disease or illness once afllicted

You kind of covered this in your last point but it's worth emphasizing: we generally don't know for sure whether "correcting" a low serum vitamin D reading helps out in some way or merely silences a warning beacon that should have been traced to a more serious underlying condition.