I think that if they draft a RB at #60, that one of the existing guys is more likely to be the starter in 2015. Coleman is about the only one that is truly good in pass pro. McFadden is terrific in pass pro and Randle became good at it in 2014. The coaches made pass pro William's #1, #2 and #3 priority in practice since he got here.
The draft pick should eventually evolve to be the starter but it's not a high probability this season. Look at Murray's stats and how they evolved over the years. He was not just an instant 1800 yard runner.
If the Cowboys are really doomed without a new RB, then they probably needed to trade up in the 1st to get one of the top 2.