I think that the fiat money won't disappear because people still need it. It's very difficult just to start use btc in just one day, but I have a good feeling that more and more people will use it in a daily base until then
I agree with you fiat money wont dissappear but if more people started to use bitcoin then the price will go down or up it can down because so many people have it and it can go up because
so many people want it to have it.