I think that the growing trend is that countries will try to incorporate crypto into their fiat currency more and more, in the form of national cryptos. Just like what Venezuela has done, and India and others are looking at doing.
To me, I don't think that it's very likely at all that fiat currencies will ever replaced by decentralised cryptos because the government would simply never be willing to give up their control. Centralised cryptos could potentially be issued though in lieu of paper based fiat currencies in the future.
However, there will be a place for decentralised crypto in my opinion and an extremely big one. The most likely thing to happen in the future is the coexistence of decentralised cryptos and fiat, with many merchants that would accept both currencies.