Such a subtle joke that made me laugh more than I should.
There is no way that bitcoin will ever replace fiats or even make them worthless, even in 20 years, this is impossible, the price of bitcoin may rise more than $10000, but it will never replace fiat. And yeah, if fiat will be replaced by another coin, what will it be? There's nothing that can replace money physically.
I guess if fiat is to replaced by something that has a physical form, it would be gold. Back then, people are already using gold and up to date, gold still has actual value. If people knows that fiat money is just paper, then they would start investing all of their fiat money to stocks, precious stones, minerals, etc. Basically things that actually have value and are a good store of value. Unfortunately, people aren't just going to be informed that fiat isn't backed by gold or anything anymore so you're right, fiat is irreplaceable. But, who knows? Bitcoin might have some spike in its adoption at some point.