The market is changing for good quarterbacks. Romo is probably the fifth-best quarterback in the league right now (there are five quarterbacks with a career passing rating north of 95: Brady, Manning the Elder, Aaron Rodgers, Steve Young and ... Romo); if Dallas didn't dump money on him, another team happily would have.
The Cowboys have an awful offensive line and a non-existent running game. Their pass rush is completely and solely dependent upon DeMarcus Ware. If Romo isn't having an ace day, they lose, plain and simple. If he plays a game that's merely middle of the road, it's not enough. As of now, the entire team's success depends on Romo turning shit into gold. It's rather amazing that they can pull eight wins a season out of it.