I agree 100% that the line is weak right now. How can they be a cohesive unit when two of their starters are rookies and completely unfamiliar with their teammates.
To be effective, an offensive line must have cohesion, the ability to perform in sync with each other. That takes more than a single season. The best offensive lines have a core of 3 or 4 guys that have played together for several seasons. They know what each other will do under certain circumstances.
However, no matter how effective an offensive line is, the RB must have the ability to know how to use the line to his advantage. He must have the vision, the escapability and the power to maximize the advantage of his blocking.
Despite having three All-Pro O-lineman in 2015, the same ones that helped DeMarco Murray set a new single season rushing record for the Cowboys in the previous season, Joseph Randle was completely ineffective.
However, the next season in 2016, those same O-linemen helped a 2nd Cowboys running back in three seasons to lead the league in rushing.
There are at least three aspects to the success of a running game. They are the ability of the RB, the ability of the O-line and the gameplan prepared by the coaches. All three of these aspects are not at an acceptable level for the Cowboys even before the season began.
The frustrating thing is, it didn't have to be this way.