One of the main problems with Keeping Romo as the backup (assuming he would even agree to this in the first place) is the media attention it would bring, especially if Dak hits a sophomore slump.
First game Dak loses, the media will start asking "should the Cowboys start Romo"? Then what happens when Dak loses a second game, a third game, what if there is several loses in a row? If this team lose 3 games in a row with a healthy Romo standing on the sideline, the media storm would be unbearable. Every TV talking head in the nation, every sports show, every radio show, every internet blog, ect.....would all be asking "should Romo be inserted back into the lineup"?
Next step is the media starts asking the players if the team should go back to Romo, what do you think Witten would say?