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It requires more infrastructure than that needed to set up recharging stations worldwide for electric vehicles. This is true even if the currency is 100% digital at all levels.

The EU wants to add in offline use for its CBDC (euro). That is another layer of integration into something like SMS.

Going to take forever.

Not only infrastructure, but also a vast amount of time.

I don't think there will be another world reserve currency after the dollar finally collapses. Rather, we'll enter into a decentralized world where different local areas adopt different cryptocurrencies.