Injuries, wear and tear from this past season, age over the life of the next contract, his lack of a 2nd gear in the open field. I don't know what his deal is when he breaks clear.
The smart move is to turn the page to the next back. The media and uninformed fans across the league will make a big deal about it. How can the Cowboys let the leading rusher go? They won't contend now! But 3 years from now people will look back and say signing Murray would have been mistake, and as long as the Cowboys can find a quality back to replace him, they benefited from letting him go.
It's not just Murray vs. the RB who replaces him. But also what you do with the money you save not re-signing Murray.
Do not make the Marion Barber III mistake again.