In the online world nothing is possible but bitcoin/Eth replacing fiat will take a long time because it has to adapt to existing rules, fiat is currently used in real transactions around the world while bitcoin and Eth are only digital currencies that can be traded. use it in online form only so it's very much different if we synchronize to replace it even though it seems that the movement has seen many countries have accepted bitcoin as a transaction tool
Even we are going all online but still it is not possible because we all know Bitcoin/Ethereum both are some complicated things and have serious issues, just because of this they can't replace fiat in long run future. Mostly governments have good benefits from centralized things and this crypto is completely decentralized which is not helpful for them, so they never allowed this all.
Fiat is not at all easy to replace which means replacing the central banking system globally which is not easy thing for many countries due to some serious issues and regulations which are completely different in many regions. Bitcoin has potential and will be seeing adoption soon, but fiat will also be in use as it is working currently around the globe.
The main fact that the government are still in control with fiat system and most of those rich business
owners continue to support fiat, it will be hard for bitcoin or Eth to replace the old generation system,
but, future is not ours to conclude we don't know what's playing inside those big players / whales, maybe
they are also considering this system, we already have them inside the market, institutional investors are
investing now with crypto we never know what are the plans behind.
Despite all that I still like bitcoin and eth being an alternative here.
as you said there are still many people who support diat and fiat is still a very strong one for now, on the other hand this I don't think is a problem because indeed crypto especially for btc and eth will still be able to continue to grow.
on the other hand I don't really care whether they will be used as legal tender or not because indeed I will still use these even if they are not legal tender and are only options.
the most important thing for now is that they need recognition from all parties, especially the government in order to facilitate adoption