One of the other reasons God of War would find it difficult to get rid of the fixed camera is that it's integral to the way the game handles hidden chests, feathers, gorgon eyes etc. Basically, go in the opposite direction from where the camera is showing you to go, and you'll get prizes.
I don't think getting rid of the camera would improve the games much. I think they're victim not only to franchise fatigue but also the fact that every third person action game plays a bit like it now, except God of War is now the one taking inspiration from elsewhere (the new traversal mechanics in Ascension, for instance, ripped straight out of every Uncharted-a-like we've seen over the past few years). The novelty has worn off, and even an impressive set piece is still basically the same thing we've seen in all the other GoW games.