> If you think it useful to keep cash stuffed in your mattress in $500 notes, and there's a massive disaster that shuts down all the banks... where are you going to convert that $500 into everyday spending money?
Since the Fed, which issues currency, is, in large part, the banks themselves, if a disaster causes a general banking collapse, you've got bigger problems with the value of your cash than whether you can convert denominations.
> all the infrastructure be fucked and ATMs don't work
Which is not that far away. A few months ago, in my town, underground workers dug in the wrong place and cut a central internet connection, and ATMs and other cash terminals in the better part of the city center stopped working. I luckily carried enough cash, otherwise I wouldn't have been able to do my shopping.
Since the Fed, which issues currency, is, in large part, the banks themselves, if a disaster causes a general banking collapse, you've got bigger problems with the value of your cash than whether you can convert denominations.