I don't necessarily think everyone's coffee purchases will be on the blockchain, but could see all the prices being in bitcoin.
Well what will they use for coffee? Coffee seems like a good thing to purchase with btc, though if it isn't obviously some fiat coin will still need to be used.
They'll still use btc, sort of. They just will use an off-chain processor. Coinbase does that today (you can send bitcoin from your coinbase account, to someone else's coinbase account, and the transaction never appears on the blockchain and it's much cheaper and faster that way - just involves more trust).