What, exactly, "backs up" the US dollar other than the full-faith-and-credit of a corrupt US feral government, which at this very moment is running the printing presses and stuffing their pockets as fast as they possibly can?
There is no limit to the number of dollars the feral government can print, and they fully intend to print a limitless supply of them.
There can only ever be 21,000,000 bitcoins, and 18,000,000 of them have already been mined.
We bought a Maytag washing machine at Lowe's last year for $500.
That same exact washing machine is now $750.
Surely you see what's coming?
“What, exactly, “backs up” the US dollar other than the full-faith-and-credit of a corrupt US feral government . . .”
The authority to tax wealth and the power of state violence to enforce that authority. The government can not only print money it can take anything from you it wants to take.
I’m not suggesting that you like that answer; but that is the answer.