While there are a few players who are exceptions, or at least making a valiant attempt to be, the notion of the bell cow running back in the NFL has largely gone by the wayside during the current evolutionary cycle of the game of football at the NFL level. There are too many specialized roles, and too many snaps to take, for almost any one single player to wear every hat every time.
In other words, teams needs more than one skilled running back, even if their skills vary from one another. The Pittsburgh Steelers are hoping that they have at least two of them in James Conner and Jaylen Samuels, a couple of their draft picks from the past two years.