Lack of sustained success is a symptom of the organization as a whole being mediocre. If you luck into a good QB (which the Cowboys did in 2006 and 2016), there is no excuse to not be contending almost every year short of a fluke rash of injuries to top players or something.
Since the Seahawks got Wilson, their WORST season was 10-6. The Packers under Rodgers have 7 10+ win seasons. The Packers with Roethelisberger have 9. That's the kind of hit rate the Cowboys SHOULD have, but their coaching and front office haven't been up to snuff since Johnson left so... yeah.