The only way you can see if someone is a draw and see how the ratings are.
Without Cena I feel people would slowly tune out.
Maybe some of the kids might but there has to be a reason they turned the TV on in the first place. They just happened to gravitate towards Cena, if he wasn't there it would of been someone else, just like if he left.
Most adult who watch it and not because their kid wants to, watches it because they love wrestling not an individual person. Sure they like someone better than someone else but I doubt they'd stop watching because someone they dislike is champion.