That's the past and it's genuinely irrelevant to what's going to happen to bitcoins because they are totally different things in different times. Gold was replaced by fiat because it's pretty hard to transact gold. It's heavy. Fiat was made but it used to have a gold equivalent. Every fiat back then can be converted exactly to the amount of gold available. After the Nixon shock, that's not the case anymore. Fiat has just become paper. The concept of money has become visual.
Now, get this. You're just holding literally paper if you're holding paper money. If everyone's educated enough about economy, they'd realize that right away and switch to something that's actually built with supply and demand or has an actual purpose in this world like gold. Bitcoin has a value because of its use, gold has the same reason although both of them have different applications.
tl;dr: Fiat can easily be replaced by bitcoin if people are educated about economy enough to realize that they are currently just holding paper.
Well , let me see if I get this right.
I hold paper money which by your view it's plain paper.
But I can use that useless paper to buy gold and bitcoins, groceries and cars.
Does that mean I'm buying useless stuff and bitcoin and gold are just numbers and metals with no value?
Of course not.
Paper money even if it is fiat as long as it has a value it is valuable.
If you consider it otherwise please mail me all your useless fiat.
And I promise you I'll be using a part to buy bitcoins if that makes you happy.