Even if un-backed fiat currencies are replaced, government issued currencies aren't going away.
I think we'll go back to some variant of the gold standard, where legal tender is pegged to hard commodity value. Decades from now, I could even see government money being backed by BTC.
It is obvious that once the government losses control of the economy they are going to try to gain control of it again by backing their currencies with something tangible like gold but at that point in time will their citizens once again trust the government to issue a currency that it will not crash? I say this because most currencies started as being backed by gold, then the backing was withdrawn and they became fiat currencies, what will stop the government to do this again?
the market will cross that bridge when it comes to it. as long as notes are convertible to gold, silver, BTC, etc (same as the gold standard era) then the market will treat them as such.
it's just like tether.
the market knows for a fact USDT isn't fully backed, yet it still trades at $1 or even sometimes above. why? because enough people can redeem USDT for dollars that arbitrage keeps the price pegged.