I think FIAT disappearance will not happen overnight. They will be around for quite a while. The "commodity" status of Bitcoin should stay for a few years.

Once the debt bomb blows and the deflationary spiral occurs, the value of physical dollar should become very high, initially. This means that credit will be declined but not the actual physical dollar.
Deflation causes the physical dollar to be the item that can actually be used to pay off or reduce debt. So, other physical assets such as gold, stocks, bonds, and maybe even cryptos will be sold to raise physical dollar to hold or pay debt.

However, there could be a real chance for Bitcoin to become the "safe" haven since it's an uncorrelated Alpha commodity (currency). This means that while the debt bomb causes implosion of all assets; Bitcoin, being uncorrelated; should hold up steady.

The 2008 crisis was actually a deflationary crisis with housing debt imploding. Still, for the dollar demise to occur, the crisis will have to be 100x that of 2008.

It's always amazing how long a person can head towards the road to bankruptcy but take forever to do so. It won't be any different for national currencies..it'll take a very long time. The FED is able to service debts toward a hundred trillion plus! That means the patient can be kept near braindead but alive for a very long time.

Therefore, I think Bitcoin and FIAT will first co-exist until the imabalance occurs..e.g., the Central banks can no longer service their debt