Funny you mention that 4.7 yard average. Ever heard of a guy named Kevan Barlow?
He had averages of 4.1, 4.7, and 5.1 in his first three years in San Francisco including a 1,000 yard season his 3rd year. He has gone downhill ever since. I think the reason he went downhill was that he was asked to carry the load and he simply couldn't handle it.
http://sports.espn.go.com/nfl/players/stats?playerId=2628
Am I saying that Maroney is the next Barlow? It's too early to say that. But it's also to early too conclude that all you need is a 4.7 avg to be counted on to be the feature back for the next 5 years or so.
I say that a smart man buys insurance. Maroney may turn out to be alright, or it may turn out that he just can't handle the feature back role alone. When you have the opportunity to draft a franchise back like McFadden, why not? Especially when it improves the position and likely extends the life of both runningbacks in a dual back system. Recall that Maroney was always in a platoon system in Minnesota and McFadden has also been in a platoon system at Arkansas. If you platoon these guys and get 1,500 yards and 1,000 yards respectively while EXTENDING the careers of both backs, I would say that is a win win situation. Why not go for the best when the opportunities to draft that kind of talent are so rare?