Currency (whether or not you use the word "fiat" to describe it) works because people accept it. People accept the paper currencies of today for the simple reasons that (a) they are used to accepting them and (b) everybody else accepts them as well. If people aren't convinced that BTC is a real currency and it can be used to buy real things or at least can be traded (both ways!) with currencies that are "real" to them, they won't be interested in getting BTC. Nobody will want to buy BTC using other currencies and the people who will have it won't be able to do much with it except give it away cheap, or trade among themselves.