The team had one of the very worst defenses in football last year. Historically bad actually. It seems extremely unlikely that they would instantly become a "championship caliber" defense in a single off-season.
They have added a lot of talent, so management seems to finally be addressing this glaring need. However, a lot of this talent is young. Take Bohanna for example. Everyone is thrilled to finally have that "trash can full of dirt" in the middle of the D-line, but this kid was a sixth round pick. It's difficult for any rookie to make a difference on either the O-Line or D-Line, but especially hard for those drafted in the mid-to-late rounds. Hope he becomes a monster, but my expectations for a sixth round rookie are low. I do think Dallas improved the secondary and LB areas.
I agree that we really don't know what we have yet on defense, but championship caliber defenses are not built in a single draft. A healthy Dak combined with a healthier O-line might aid the defense more than a bunch of rookies. If Dallas can control the time of possession more, and also make other teams play from behind, it will help the defense tremendously.