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.