I said this elsewhere, but it's relevant here too:
The idea that they might be giving Cena Ziggler's briefcase because doing so is somehow going to prevent Ziggler from becoming a credible, long-term main eventer really holds no water. Ziggler doesn't have to be a champion of any kind to be a main event talent. All he has to do is feud with the right people and either go over on them or else get enough 'rub' from them that he comes out looking like a winner even if he didn't win. This feud with Cena has gotten him as over as he's ever been, and even if he doesn't emerge from it victorious, he'll still be in the main event picture in some fashion.
Given that he's already lost to Cena once, I personally think he'll come out of TLC with the briefcase, but it won't be the end of the world - or the end of his relevance as a main event-caliber guy - if he doesn't.