Dude, that totally would not have reeked of awesomeness.
Besides, if he allowed Rollins to kill Edge, Seth would have just grabbed some Make-A-Wish kid and threatened to kill him.
And Cena declines. Swerve! Heel turn! Curb stomp on a child and Cena laughs. The media would report on that for a solid week. Bam! New eyes on the product. Buyrates go through the roof as people tune in out of curiosity at first, then over time to see who will bring that dastardly Cena to justice. (Seth Rollins probably died in the riot the curb stomp caused.)