> There are doctors that are prescribing 5000IU+ daily

What doctors prescribe based on the actual conditions particular patients exhibit (and, at doses that high, often with serum level monitoring) is only distantly related, even in theory, to population-wide recommendations like the RDA.

> I've read lately about multiple papers that propose multiple k of IU per day.

Yeah, so have I, and I personally think (though I’m not an expert in this field) it's likely that the RDA is way too low. But there's a difference between the belief that RDA should be higher and the claim that the RDA has, in fact, been raised. It has not, it's still 600 IU/day.

Doctors may be making those recommendations based on blood tests indicating specific patient deficiencies. As you note, though, research does at least suggest daily vitamin D supplementation in the thousands of IUs per day range may be considered safe.

I'm not a doctor, but I believe if you've been directed by one to supplement vitamin D, blood tests probably should have preceded that recommendation.

I was told to take 20,000IU/day, just to be sure. It might be just correlation, but it helped with the issue I had.

I've had one doctor prescribe me 10K a week. Another one recommended no more than 400/day.

Doctors do not agree. Some have jumped on the bandwagon, and others believe it is just a bandwagon.