I've seen evaluators I respect say he's a top 5 talent. So I am not defending a hill on this or anything. I am simply not so sure that he's going to be valued at that level by teams.
Rationale is that while he has some snaps showing an ability to play over the top he's most likely to produce splash plays in the box. So in the NFL he's going to be used a lot in that box role, which is not a rare skillset, in that you can find them in every draft.
Also the SS types who go earlier are usually LB sized but have that safety movement ability. Downs is safety sized.
Also the Safety position is undervalued, due largely to the aforementioned supply and demand. Unless you have something that is rare or difficult to find. Downs has great instincts and is tenacious as hell but are those enough for him to go that high? Again not sure. Maybe he has an epic workout at the combine or his pro day and that gives him an edge in that respect, we'll see.
Also this Safety class has some legit options other than him. If you want to get right at safety you don't have to take it early.
Of course all it takes is one DC jumping on a table for him in the early going. Or a front office falling in love with the prospect. But again if I wanted an elite box safety, who even has size and length and strength on top of that to hold up in the box in the NFL, I'd go Styles.