It has been a magnificent run of on-field prosperity for the Seattle Seahawks, although not exactly New England Patriots-like, but superb by the standards of the rest of the NFL. They have won one Super Bowl and were an ill-fated play call at the 1-yard line from winning another. They have been an annual contender for more than half a decade. And for most of this season, the Seahawks seemed capable of adding to that list of accomplishments, as they jockeyed for playoff positioning with the other NFC heavyweights, the notion of making another trip to the Super Bowl not entirely far-fetched. But the narrative can change quickly in the NFL, a week-to-week league in which a franchise’s entire way of doing things can be affirmed or repudiated each Sunday. And the narrative indeed has changed in recent weeks for the Seahawks, to the point that when they take the field Sunday in Dallas, there is room to wonder if this is a last stand of sorts for the team that everyone has come to know so well in recent years. “When you lose a couple games in a row, it’s no fun,” Coach Pete Carroll said at his news conference Wednesday. “That’s different. We rarely have been in that situation. … We’re grinding right now. We’ve got to grind our way back into a win.”