There are benefits to "RTFM". They may involve making sure there are shared assumptions during speech that allow further speech to be economized equitably between speaker and receiver. They may involve the internalization of the structure of a model so that they it be applied fluently in some practical setting. Your suggestion fails to satisfy either condition as the cost is either inequitable, or the benefit is irrelevant.
Putting a tag that says "moderator" next to a username is such a cheap quality of life improvement that even if what you claim is true, it really shouldn't be the job of the audience to figure anything out in the first place, as by comparison, it looks like a strong disfavor.
Words to avoid living by.
I don't see this as RTFM; there's no FM to begin with, other than perhaps https://news.ycombinator.com/newsguidelines.html and https://news.ycombinator.com/newsfaq.html.
What are those 'downsides....quite strong downsides actually'?