I think his gimmick plays a part, but I generally agree with you. Cena is the unstoppable, superpowerful Superman that all the kids want to be. Comfortable speaker, fan-friendly, hard-working, booked well. He's like a meal high in nutrients, but that still needed charisma and an interesting gimmick to put sugar on the food that was being shoved down our throats. That's the difference in why Sheamus didn't get over and he did.Cena is a very hard worker, and does absolutely everything possible to benefit the company. I don't think his gimmick has anything to do with why he is the face of the company. Thats my thoughts although I may be wrong.
P.S. You're sig made me remember what's important in life.