I don't think anyone thought it was an elite defensive roster coming in. Injuries have mattered. But I also think you can't discount the coordinators, on both sides of the ball. And clearly some complacency (and arrogance?) coming off of last season.
But, we're still talking about a likely 13-4, #2 seed with four quality wins and three losses to quality teams (all in a row before this one, as it happens). And last year's team had the playoff luck (facing a bad team in the first playoff game, the injury to Purdy). This year's still could. And none of the other teams in the NFL look especially perfect. For a while it seemed like the Niners were but now the Eagles still have a chance at the top seed. Maybe the Ravens are the best team in the league.
And both of last year's SB QBs (and also Purdy) are diminished somewhat (relative to the high standard of, MVP-calber).