I've bought $500 bills and $1000 bills for "collectors value", but the understanding is that they are STILL legal tender.
Even if the government collapses.
BTW, my grandpa told me how when he bought his house in 1946, that he made the down payment in $500 bills.
He was so "awed" by that, it struck him.
Because in 1946, a single $500 bill was the result of untold hours of labor.
He guarded them like they were gold.
And his mortgage in 1946? $8000
As collectibles, they are worth more than face value. But if there is an economic crisis, I would not try to use them as currency, for the odd reason that many people do not know they exist, and would be very suspicious.
This paranoia is so pervasive that it is even difficult to use $2 bills, and younger retail clerks, having never seen one, automatically call the police. A $500 or $1000 bill? They might hold you at gunpoint.