I see what you mean about when cash disapears monero may be all thats left for the black market, and if that can never be exchanged for digital fiat on an exchange (which it likely won't) then white market goods (say food ) may start to appear on black markets, else those selling balck market goods won't really be able to live of their profits.  But still purchase of other things  like utilities, cars and houses  aren't really  possible that way . Black market sellers will still struggle to spend their monero profits on things they actually need.  Fiat isn't going to disappear, but when paper cash disappears, maybe what happens is that black markets just shrink or disapear as crypto becomes pretty useless for buying things you need to live off.   Some may see this as a good thing.  Of course any country who doesn't get rid of cash or legalises monero as currency could cash in on this, as El Salvador is doing with bitcoin.  Then again they risk get invaded or overthrown in a coup de tat via the US foreign policy.  As we have seen in many countries that have attempted to change their currency or petro dollar to something else. eg Libya..