It's not even a problem if they run out of imaginary money because the fed can simply ease in more to the FDIC via debt shell games.
Your local bank has a 0% reserve requirement[1]. Ofc it'll most likely get bailed out by the FDIC (by taxpayers) in case of emergency but US regulated stablecoins are in no way more risky than US banks
1: https://www.federalreserve.gov/monetarypolicy/reservereq.htm....
I'm not particularly bullish on crypto, but this is definitely not the flex you think it is.
There are incidents where a national currency winds up getting devalued, like when Soros bet against the UK pound. But proportionately that's a small decrease in value, the pound didn't go to zero like failing stablecoins do.
It is only a question of time until it fails like everything else does, but so far so good.