Cena isn't going to get another monster year-long reign as world champion. 6 months max. He's still the face of the company, but's primary focus is still to create new main-eventers. He's still holding his top spot because WWE has YET to find a guy that can:
1. Get over as a main-eventer.
2. Put new asses in seats.
CM Punk is like Randy Orton. He got over as a main-eventer, but he dosen't put new asses in seats. His fanbase is the IWC and people that will watch WWE regardless of how bad the show is.