Cena getting hurt could have been used to push younger talent and get them into main event status, but instead they've used it to try and get Reigns over and have failed miserably. People don't want to be told who to like, they want to like who they want to like. Which is why Daniel Bryan was so popular. And WWE was pretty much forced to give him the title.
Cena does nothing but hinder the younger guys from becoming stars. There is a reason that when someone faces Cena, they all of a sudden lose a lot of steam. Wyatt and Rusev are the two most recent I can think of. I don't like that he is hurt, but its been pretty nice with him not around.