I think you are thinking of a position of Sherfield can not be replaceable. Fournette adds a different dimension. Sherfield's snaps if all he does is block can be replaced by any one of our TEs, he's not a threat if we never throw him the ball. Again the position is extremely interchangeable with a different player regardless of player. Don't get caught up in what players position numbers are, rather how do they add a dimension to this offense. Harty gives the potential of a deep threat that no other receiver does but Sherfield does not provide anything unique. Fournette gives the experience of being a super bowl champ so when the game is big he can rise to the occasion no matter the situation. Also if you want to look at last season they used 4 RBs (not including Gilliam) every game last year. The position that was short though was Tight end. So you could look at it like Sherfield would be replaced by Kincaid.
I believe he has skills but I think he would be bigger for special teams