The Dallas defense ranked #32 last season. What happened the last time a Niner offense faced a suspect defense in an opener? Kaepernick dropped over 400 yards on them. And the Packer defense was actually a lot better than the one the Cowboys have now.
The only decent defense the Cowboys faced last season was the Saints - they scored 17 points on the Saints.
The Niners should win this game. You never know for sure in the NFL, but the Niners should win.