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
that is right that fiat currency will continue to exist for a long time and people will be using both bitcoin as well as fiat currency for shopping and all other purposes, hope that in 2022 the number of bitcoin users will be too much as compare to the present number of bitcoin users.