As long as he has the belt, his options are limited. With the UFC's "Champion" clause, they hold his contract even after his fights are done (he has 3 left, IMS).
The UFC has pretty tight control over all their fighters, but especially the champs.
He can't just walk, because unless the UFC decides to let him, they still old the reigns. Couture proved you can't just walk away and do somethign else.
All I'm saying is that Silva, whenever asked, seems to want to retire, or go do other stuff that the UFC won't let him do while he's the MW champ (superfights, RJJ, etc). He could retire, but only if he completely walks away from the sport, since the UFC will still have control over what he can or can't do.
I'm not saying I think Silva's going to throw the fight. I'm just saying that the thought has crossed my mind. He may well decide that he'd rather have Maia, a countryman he obviously likes and respects, take his place if he decides that he wants out of his contractual obligations.