Saying that stablecoins will replace fiat is like saying that gold certificates will replace gold.
Discussion
Gold certificates did replace gold.
Scam
I actually do believe they will.
Their needs to be a privatized co-equivalent to offramp civilization from tradfi fiat as we deconstruct the old arc and remove money control from the politicians/central banks' court.
Stablecoins or any CBDC will fail (miserably and predictably) if it changes its inflationary schedules, etc; in essence depegs itself from its master. Bitcoin. Which will be the Reserve currency for everything/anything any society would like to use as a digital currency for transacting.