I don't think fans are scared as much as they are frustrated.
The Cowboys have played games like this in the past and they won some and lost some.
The wins did not guarantee season successes and the losses did not guarantee season failures.
If the Cowboys beat the 49ers Sunday and then lose to them in the playoffs, did this game really matter beyond helping the Cowboys make the playoffs like every other (NFC) game they won?
If the Cowboys lose to the 49ers Sunday but beat the 49ers in the playoffs (meaning the Cowboys make the playoffs), did this game really matter beyond hurting the Cowboys post-season seeding?
The only way this game really matters is if the Cowboys lose the game and it prevents them from making the playoffs or it costs them the division or #1 seed in the playoffs.
As a Cowboys fan I want them to win every game of course and NFC opponent games are the second most important games they need to win after division games.
That said, if the Cowboys beat the 49ers Sunday it means little really beyond an NFC win for the Cowboys and an NFC loss for the 49ers, both of which may help with playoff seeding later assuming those teams make it to the playoffs.