Why would it be a "huge fail?"
IMO the whole "thinner" thing is a gimmick and it's getting old. Every year the iPhone gets thinner. It's nothing new and not a good reason to buy the iPhone over any other phone. And when you put a case on it, the whole "thinnest iPhone ever" becomes moot.
You can't be serious. The length and width of the iP6 Plus with a thickness of the 5S would basically be like toting a brick around. Even most tablets are thinner than the 5S.
Making devices thinner is not a gimmick if they are also increasing length and width.