Fiat currencies may collapse but it won't make fiat currencies disappear, there will still be many who use fiat currency for transactions because there are still many people who use fiat currencies and not everyone understands the use of currencies other than fiat currencies.
Not possible the least. Fiat is one of the most regulated currencies by the government and this is the reason that despite the devaluation in the value, it does not get the demand decreased.
Special cases like that of Venezuela have been around which were a combination of both hyperinflation and huge devaluation has resulted in a lowered demand for fiat but that is not the case everywhere. Fiat will only be out shined if the technology becomes as cheap as holding a note of one dollar that is accessible to all.