Don't let yourself forget the key reason why teams trade up; a player drops. In a draft without elite talent, that's particularly likely to happen. Some team in the top 15 will pick a guy who team x had graded out as a second rounder, and consequently, there will be a guy availiable at 20 who team y never imagined would last that long.
That's when teams trade up. They don't do it now, they do it when a guy falls that they've been salivating over.