If there is mass acceptance, dumps won't be necessary. He won't need fiat.
A lot of people don't seem to understand that if Bitcoin is successful it will be
the money - currency trading as an industry and occupation won't exist any more because there won't be other currencies to exchange.
That is just idealistic bullshit.
Bitcoin can take a major part in the world without destroying fiat completely. There can also be other cryptocurrencies besides bitcoin.
Bitcoin will definitely never be the only money on earth.
I don't see any reason for fiat to stick around, unless it's by force. Let's imagine bitcoin is accepted everywhere, what possible reason would you have to accept anything else? Perhaps a different coin is more anonymous or has some technical feature. But bitcoin could probably copy those features. And there's always off-chain transactions (which still use bitcoin as the unit of account).
I don't necessarily think everyone's coffee purchases will be on the blockchain, but could see all the prices being in bitcoin.
Some months ago, I met a friend of mine, since he was in my city. He was there, because he had to deliver some blue prints and a 3D-model to a architecture-competition.
I asked him, why he couldn't just send them over the internet and he gave me a lot of stupid reasons like the line width of the blue prints depend on the printer or that it has to be a closed letter so nobody else can see it, yada, yada, yada.
I wasn't able to convince him, that all this things could be guaranteed by sending it over the internet if just somebody took the time to implement something like that.
So, that is the reason, why fiat will not disappear. It is never just about, what the best solution is. It is also about what ignorant people understand and what they are willing to change.