Two and a half years off main-eventing Wrestlemania, and he's jobbed to Brodus and Kofi, plus that mini-feud with Ryback- I'm guessing he jobbed but I haven't kept up other than knowing Ryback chucked a couch at him. There is something seriously wrong with that booking, and if I'm Miz, I take my chances with TNA when my contract is up.
He'll make more money jobbing in the WWE, especially if he gets a PPV spot here or there, plus merchandise.
He's not the type of guy i'd suggest to go to TNA. Even at the top in TNA, he figures to make more as a mid carder in WWE, plus there is always the possibility they decide to push him again at some point.
Guys go hot & cold, for whatever reason he's cold right now. But just existing on the roster and getting PPV checks and ending up on DVD's or whatever, he comes out ahead where he's at as opposed to TNA.
TNA is great if you are looking for your first break (Magnus, Robbie E, etc), or if WWE doesn't want you (Chavo, Chris Daniels, etc), or if you are a veteran and want the easy schedule (Sting, and people in the past like Booker T). Christian is one of the few I can think of who truly jumped from WWE to TNA on their own, while in his prime. And his was a case of being frustrated creatively, and also wanting to dabble in acting. Most don't have the guts to do that, and in the long term, it probably cost Christian a shit ton of money.