Once a certain currency becomes "worthless", the responsible central banks under the respective government directions will issue another.Have you even been aware that countries like Brazil changed they fiat currency many times when one became "worthless"?
So far in history we've only witnessed cases of certain countries' currencies going to dustbin and resurrected with the loans from international financial organizations willing to provide those loans to countries, but what I am talking about in my previous post is the situation when the reserve currency itself is going to dustbin, who would back up other countries' currencies then? That's when your fiat wouldn't buy you a cabbage, actually you might even get your face punched for offering to pay in fiat for that cabbage

I don't even want to comment on your 'responsible central banks' phrase, it's too funny, I wouldn't know where to start.
It's actually been decades now that the reserve currency has been depreciating in real purchase power terms, since Nixon canceled the Bretton Woods system in 1971. Since then all currencies have been living on borrowed time.
When I said 'a few years', I meant the final accelerating hyperinflation period, when hyperflation becomes noticeable to everyone, who goes shopping for groceries.