Bitcoin dominates --> Regular John sees writing on the wall as fiat currency crashes --> Bitcoin is too scare and pricey to get any amount equal to the dollars they've saved --> large companies, governments, and early adopters become hyper wealthy --> major wealth gap and misery for most of the world except us --> sad state
You forget that regardless of the attractiveness of a single asset (in this case Bitcoin), people will still need the economy to satisfy their needs.
If the economy is doing well, then "Regular John" will have his job to be able to save money, and even if he loses some savings in a "fiat crash", he can probably recover them. Many of us here hope that Bitcoin will have positive consequences for the economy, such as less friction due to inflation, increasing the global standard of living. And thus "Regular John" will benefit from it too.
Besides of that, I consider a worldwide fiat crash unlikely. And even in that case: Most people don't save only in fiat. They save using stocks, real estate, ETFs, gold, and increasingly Bitcoin. Besides from fiat, all these goods will probably appreciate in a fiat crash, even if perhaps less strongly than Bitcoin.
This scare tale last year was also distributed by two well-known Central Bankers of the ECB (Ulrich Bindseil and Juergan Schaaf) in
this text. For the reasons mentioned above, I think they're wrong. Just don't put
all your savings into fiat.