One thing I've seen is that some RBs may have 1 big season, but if you look at the guys who produce year in and year out, it's a pretty short list. Look at the rushing leaders from say 2013-present, and you'll see some Darren McFaddens and Justin Forsetts come and go, but guys like Adrian Peterson, LeSean McCoy, Marshawn Lynch, Gurley, Bell etc, are usually floating somewhere around the top 10.
That year to year consistency is what you buy with an elite RB, in my opinion. Guys who can take 300 touches a year, remain healthy, and churn out yardage rain or shine don't grow on trees.