In a grocery store I pay with dollars coz noone accepts bitcoins. In 20 years the situation will be the same. Because some guys made a bridge between dollars and bitcoins. Because they hid a wallet with coins in a bank vault and gave ppl paper money. Because they made Bitcoin an instrument to store wealth.
Baby steps. First the merchant accepts a card that converts BTC into dollars. From there it's just a tiny step for the merchant to use his own service that accepts BTC and converts it into dollars (like BitPay). This would be good for the merchant, because instead of being locked into VISA or MasterCard, the merchant can accept almost-fee-free BTC transactions, and chose from a very large selection of competing exchange services to convert the money for him. After that it's just another tiny step to simply keep BTC.