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I wouldn't say it can't be done with national digital currencies to create a network that let's you send money within no time across the world for an almost zero fee. Technically, it can be done. It does already exist for the Eurozone, the so called SEPA network. You can send for a fee money to any country in the Eurozone within 10 seconds. Bitcoin has other advantages than being borderless when it comes to transactions. It is also borderless when it comes to taking your wealth with you because nobody knows.
I am not from that region and this is the first time i am hearing about a separate SEPA network for European countries and what is the transaction charges and when did they implement these, i bet after the Bitcoin market came into existence because for SWIFT transaction you need to give a minimum of $15 and upward depending on the amount you are sending.