There are still many parts of the world, including stores, taxis, but also people with smartphones, that do not have Internet. It is doubtful that even in the long-term future everyone is going to have internet on their smartphone.
Who knows in future, there will be free/paid internet access across the globe! big companies are trying to do that, one such project is called "project loon".
So how is bitcoin going to work for retail transactions? Imagine a store without public wifi. Can transactions be processed if only the vendor has Wifi?
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There are already some devices/apps that let's retailer accept bitcoin payment via bluetooth/NFC. in coming days there also going to be bitcoin cards (like the normal ones) that can be loaded with bitcoins/altcoin and be used in any stores.(bitcoin fuze card?)
https://www.coindesk.com/bitcoin-box-can-process-payments-web-connection/https://themerkle.com/4-companies-providing-bitcoin-point-of-sale-terminal-payments-to-store-owners/