I wondered this for a long time. It just seemed they could not come up with a real, and decisive argument. Then, at some point, it became obvious that 2 things tended to happen. First, people would have all kinds of arguments about why it would not work, then if those arguments were applied to fiat it suddenly made fiat appear weak. Second, game theory, and by people spending time finding the weaknesses of Bitcoin, they actually found it had strengths were fiat had weaknesses.