Firstly….I have zero issues on the creative direction that WWE chose by putting Cena back in the key spot. They know, as does John, that Cena doesn't need the title to sell tickets, PPV's, or merch. However, they also know that Cena is the perfect talent to put the title on someone else, I'm still advocating Brock Lesnar at Summer Slam, which will then facilitate Lesnar passing the title to a younger upcoming star (Roman Reigns at WM31).
I would also speculate that many within WWE including Cena himself knows that John is over exposed but main event talent depth is not overly prevalent in today's WWE and it's a process that takes time. Establishing new stars of which we are seeing with talents like Ambrose, Rollins, Reigns, Bray Wyatt, Harper & Rowan, Big E, etc isn't an exact science and generally takes multiple years to accomplish.
The key to developing new main eventers is making sure that everyone is evolving and not living in their own, personal comfort zone and that starts with every member of the roster with no exceptions.