To me it all comes down to health. Because of how this team has drafted the past couple of years (trading up and losing picks from first 3 rounds), they have poor depth. Most teams don't have great depth but I think losing 2nd and 3rd round picks year after year have really made the Cowboys top heavy. Injuries happen in the NFL, some teams can weather it, I don't think the Cowboys can.
The position groups that I feel remotely okay with if Dallas were to lose a starter are RB and WR. Running backs Randle, Dunbar, and Williams (on PS) I think could fill in adequately if Murray were to miss time, the only question would be how those back hold up in pass pro. Dez Bryant is a beast in the redzone and there isn't anyone on the team that can make the catches Bryant makes, however I feel there are good enough options for Tony (Williams, Street, and Byrd) that they could make do if Bryant were to go down.
I think every positional group on the defense has poor depth, and I am not really in love with the depth on the offensive line either except for maybe Bernadeau who would fill in if either Leary, Martin or Frederick go down.
The common theme for the past three 8-8 seasons were injuries at every level of the defense and not good enough depth to make do. It may be a little different for the defense this year because there are players that will be able to help after they reinstated. Demarcus Lawrence at right end, and Amobi Okoye and Josh Brent at defensive tackle. Garrett has also made a concerted effort into investing in injury prevention technology to help limit soft tissue injuries that would plague the Cowboys of old.
As far as the defensive play and will it hold up? Probably not, I do think that the offensive execution is helping the defense. Dallas was last in the league last year in converting third downs. They could not sustain drives and it resulted in their defense being on field longer to be exposed. This season not only is Dallas executing on third downs and sustaining drives, they have an effective rushing attack to shorten the game.
I will say this about the defense it seems Marinelli is utilizing different blitz packages to make up for consistent pressure from his ends. And the defense is a lot more disciplined when it comes to its run fits, zone assignments etc.