I really, really hope your right. At the moment, my gut tells me the Cowboys struggle early on in the season as they relentlessly try to run the ball...only, it will not be as effective as it needs to be. Randle may be able to get more yards than Murray in certain clean situations, but Murray was good for at least a yard when he was met behind the line. Those runs are critical.
The defense will be hamstringed to a certain degree by the lack of Hardy and McClain. Granted, the Cowboys have good talent behind these guys, but there will still be a drop-off in talent.
I think this Cowboys team will find a way as the season progresses, but we may not see domination like the whole of 2014 until towards the end of the season, when they find their stride.
Here's the thing: we were still pretty effective offensively prior to 2014, and Randle, at the very least, is a reasonably capable NFL RB if you look at just his effecitveness last season and know that he's running behind a very good run blocking OL. I think we at least get the running game effectiveness we had prior to last year. Plus, we're going to finally (famous last words) incorporate more screen games and plays in space (I say this because Randle and Dunbar and DMC are all good receivers, and the team keeps saying they want to use Dunbar more, and he's basically primarily a weapon in the screen game). McFadden can pass block, at the very least. And we've got Beasley and the two TE set options.
We're just too solid a passing team for us to be shut down offensively. They have to play the pass, and we've got very good run blockers. No matter what happens, I just can't see it being all that bad.
On defense, we'll start the season with Lee replacing Ro. McClain. That's still an upgrade. And we'll have Demarcus Lawrence, who we didn't have to start the season the year before. Plus we'll have Byron Jones. We won't start as good as we're going to be, but we won't be much behind where we started last season when Hitchens was a rookie and McClain came in late to camp and Mo went down relatively early putting Sterling Moore in the lineup.
But, yeah, I'm talking more about the whole season, anyway. I think we'll end up with a lot deeper/more effective front 7, and that's going to help us offensively, too. We'll play with more leads as a result, and put up points in the passing game and run the ball more, late. We may have to convert more on third and 2-5 yards than we did last season, because you're right that Demarco was great at getting positive yardage when the play wasn't there. But Romo can do that for you pretty well. Big plays from defense and playing with more leads (if we're lucky) is going to more than make up for some of those series where we're making it harder on ourselves on 3rd downs.