I agree with your general point, but I think you're a little off the mark. Bush, Peterson, and Johnson really are rare prospects.
That the latter two haven't played a down of NFL football in no way diminishes their ability or eliminates the chance of them becoming perennial Pro Bowlers.
As for Peterson "disappearing," I don't find it a major concern. Sure, he had some dud games here and there, but his stats were pretty phenomenal. It's not his fault that his team was horribly outclassed by USC. Granted, the Vikings aren't going to have a talent advantage over many of the teams in the NFL, but I still don't think anyone should be worried by Peterson's college production.
Steven Jackson had a pretty weak final season in college. That can happen when you play for a mediocre program in a major conference and are the center of every opposing team's attention.
I could understand Peterson being stifled by USC. I can't understand Peterson being held to 66 yards on 23 carries (2.9 ypc)... by
Texas Christian University.To be honest, I don't care how good of a PROSPECT Peterson is. I care how good of a career he's going to have. Ryan Leaf was a tremendous prospect, too, while Terrell Davis was an absolute afterthought in the draft process. Tony Mandarich and Robert Gallery were both absolute "can't-miss" guys (Gallery, in particular, was described as the safest pick in the last decade). When I'm drafting, I'm trying to maximize my chances of getting a guy who is going to have a stud NFL career, and to be honest, a former 6th rounder who rushed for 1500 yards is more likely to have a stud career than a "once-in-twenty-years talent" like Peterson. That's just simple fact, borne out time and time again by historical evidence. Guys who have proven it in the league are drastically underrated compared to unproven guys like Bush and Peterson.
In fact, Reggie Bush is the perfect example. Last season, many people valued Reggie Bush more highly than LaDanian Tomlinson. This season, people value Adrian Peterson more highly than Reggie Bush- despite the fact that Bush looked pretty darn good this past season. And as I said earlier in this thread, I suspect next year there's going to be a "Who would you trade Darren McFaddon for" thread, and Adrian Peterson's name isn't going to be on the short list.