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If it was gradual, sure, maybe. But would banks even let that happen with how much power they hold? Lol. They'd probably just incorporate crypto instead. And yes, still be as slow and as inefficient.
Even if crypto will chase the fiats to exile of no returns, I doubt if it would happen in this our generation but the future generations because currently as it is, we still have high level of illiterates who are literally not readabilities to cope with the digital currencies neither are they internet exposed.
It really seems impossible to even see this happening while the governments folds it hands watching the fiats under their control vanishes for decentralized currencies that they will not have power of control on except for having limited power of order on adoption based on regulations and as well the possessions over centralized exchanges since they are connected with the fiats.
And likely, it the CBN opts out their fiats from the crypto for assets exchange wit the P2P, that may be a degree having crypto negative influences on it usability for exchange as digital hold of values.
Note that the value of Bitcoin is visible and adorable when valuable commodities are in competition on which on poses a good hedge of values. So, there is every essence for the fiats to stay even when looses it values.
Still, fiats is impossible to go on extinction in such margins of 10 years because there is still a poor number of adoptors.