Defense in today's ruleset has changed fundamentally. It's not really about dominating and 3-and-outs. Today it's about generating a good set of 3 downs to stop the opposing offense before they score a TD, or generating a turnover.
Defenses can't regularly dominate the way they used to in a run-first league. With the rules changing to make it so much easier for the offense (protecting the QB from hits, disallowing DB's from using their hands downfield, etc.) it is a lot easier to pass, and offenses are scoring more. Defenses can't regularly shut teams down. Sure, a few teams for a few games, definitely. But on a league-wide level no. No defense can shut down EVERY team in the NFL anymore, because the variety is too great. For instance, if they have the team speed to clamp down on short passing games, they're vulnerable to the Harbaugh-esque power pig football and deep ball.
So, it's not about the DL playing every play, it's about having those elite DL who can generate enough of those impact plays so that you cause a lot of the takeaways and sets of 3 downs worth of impact plays to prevent TD's. OL is about consistency these days … the OL is expected to win consistently and is graded on how often they are inconsistent. The DL is expected to lose, and is graded on how many exceptional plays they can make an impact.