People only say that Bitcoin will replace banks and fiat because they then think the value of their coins will go up significantly, and thus the 0.1-0.2BTC that they own will make them "rich". Beside the fact that it's impossible, they don't realize what it will mean for them (in the negative meaning of the word) if something happens to threat fiat and banks. It's pure greed that makes them overlook any sort of risk that comes towards them in whatever situation. And if things don't play out as they were hoping for, instead of blaming themselves, they blame Bitcoin...