Dallas wasn't the worst defense in the NFL last year. San Diego was worse. I know you'll laugh and point to yards or points. Dallas faced a very tough offensive schedule. 5th toughest in the NFL. So while they conceded more yards and points than San Diego, all factors considered, San Diego was worse. Getting lit up by teams who're good is expected, San Diego got lit up by the Redskins after San Diego had a bye. Where Dallas trumps SD and GB who were less efficient, Dallas crumpled late in the season.
Dallas won 8 games last year cause their OL is vastly superior to the Falcons, their schedule was easier, and most importantly, DeMarco Murray. Dallas also had an easier schedule. Dallas clearly in the last 2 years has much more talent around Romo than the Falcons do around Ryan, that's not even a little debatable.