Can't just look at the Win/Loss column and make definitive conclusions off of that. Saying that they have a bad Oline tells me you haven't watched the Cowboys this year. The Oline has been awesome this year, especially compared to previous seasons.
Dallas is an average team with above avg talent... their problem is consistency from either side of the football.
Week 1 the O essentially scores 21 points vs a team that's currently allowing 36 per. The defenses forces 6 turnovers that game.
Week 2 the O struggles again but the D plays good.
Week 3 they both play good but it was the Rams
Week 4 the Offense can only score 14 vs one of the worst defenses in the league
Week 5 the Offense scores 48 points, but the defense forgets how to play.
The offense will play amazing one week then stink it up the next, same with the defense. I'm honestly expecting the Cowboys to struggle to score 21 points vs Washington this week. But it wouldn't surprise me to see the D play great. That's coaching. If they could get a consistent performance from either side they'd be a 11 win team.
They have the talent, and they show flashes of that talent every year like DWS said, but until they're consistent(on offense or defense) they'll be average.
And being consistently average means consistent middle of the pack draft picks, which leads to more years of being consistently mediocre