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If the US government still exists in 20 years -- and I'm betting it will -- the dollar will be around, too. It may be worth much, or it may still be the most valuable currency in the world, but it will still be here. Cryptos won't replace fiat, they'll be an alternative to fiat. I expect in 20 years, you'll have a choice of whether to buy in fiat or crypto, with some cryptos having special functions. E.g., you can pay in LTC or USD, or pay in BTC or Yen. When you buy a house, you can pay in fiat through traditional settlement company or use some real estate token with smart contract.

In other words, you'll get to choose what currency you use.

yes, platforms like Stellar are trying to make that possible !

I could definitely see this as a future reality. It wouldn't surprise me if the choice of paying with crypto or fiat will be as natural as paying with cash or card today.