Well, they won't be leaving before it starts, but they are likely to leave as soon as the outtakes are complete at the end of the film (characteristic of almost all Pixar films save for Finding Nemo). So putting it at the end is just stupid. Plain and simple.

I agree with everyone here, no one will be buying tickets for The Incredibles just to see the SW trailer. No chance. People are going to see the movie. It's gonna be huge. I'll be there. But primarily for the movie. The SW trailer will just be an added bonus that is more than likely to make my wife roll her eyes.