With cash the cops would have to raid my apartment to get the money (also likely they wouldn't know about the money in the first place, so no cop raid there!).
So yes, uncle sam can steal the money in your mattress without having access.
Or the Fed could release a newDollar and only accept a certain amount of oldDollars. Russia (I think) has done that once or twice.
2/3 of oldDollar cash is stored outside the US by people who mostly can't enter the USA without visas etc. And those will be pissed royally.
It would be not wise choice to piss off more people than your own population, however not never done by the USA.