I certainly would not expect or want to see anyone overtake DW next year. If you look at the offense we will have next year, especially at the skill positions, his experience in the offense will be invaluable. With a new (starting) QB, two new (starting) WR's, a new TE if Carlson leaves, a green O-line and a returning-from-injury FB, having a dependable RB should not be underrated.
While Darius may not be a great running back, I think he is an excellent football player, one of the best "football players" on the team. He plays with passion, does not make many mistakes and has been as dependable as anyone on the team over the past 2 years. He may not light up the stats sheet, but you know what you are getting. Sure, we'd all like to see him break off 50 yard runs every now and then, but next year might be a good year to simply move the chains and have a dependable outlet for a young QB. DW is the right man for this job.