The season is screwed if Romo goes down. Tanney or Orton, it wouldn't make a difference other than the big money Orton is getting.
Just about every team in the league would be screwed if their starting QB goes down for all or most of the season especially if their starter is a top 10 QB. Orton has been a starting QB in the NFL he's much better than Tanney which is why he has a job with the Cowboys and Tanney doesn't. Orton's starting experience gives the Cowboys a chance if Romo goes down for a few games. If Orton had to take over for an injured Romo the situation won't be too big for him because he's been a starter. Old man Kitna was able to go 5-3 after Romo went down in 2010 with a team that wasn't nearly as talented as this team because he had a lot of starting experience. Tanney wouldn't stand a chance in a regular season game but Orton would give the Cowboys at least a chance to stay in the race until Romo returned.
Orton certainly isn't anything special but he has won some games in the NFL including a win over a 13-0 Packers team in 2011 that was being led by the league MVP Aaron Rodgers so I wouldn't sell him short if he's needed for a few games. If Romo goes down for more than 5 games then the Cowboys are probably done but the same goes for NE, GB, NO, The Giants and several other teams because there's going to be a huge drop off with their backups. It's hard finding a starting QB who can win games consistently much less a backup who can win games. You're just hoping your starting QB isn't out long and your backup can keep you in the hunt until your starter returns.