Fiat can never be worthless even if the value of bitcoin reaches to the moon
Fiat very definitely CAN be worthless exactly as the vast majority of fiat currencies created are now worthless. EVERY fiat currency ever used has eventually become worthless since the dawn of time except those currently in circulation for now. If the fiat currencies currently in use DON'T eventually become worthless then they will be the first ever in history. I believe that in time ALL fiat currencies will become worthless just as all before them and new fiats will come in and have percieved value for a time before they too become worthless just like all the rest have.
If 1 BTC = $1,000,000,000,000,000 then the $ is still not completely worthless because it is still worth 1/1,000,000,000,000,000 of a bitcoin and therefore is worth
something.
When 1 BTC = an infinite amount of $$ that is when $$ are truly worthless because then even 1 satoshi is worth more than all of the $$ in the world.
In this case it still doesn't mean that BTC has gone up as X amount of BTC is still worth X loves of bread, same value, but now an infinite amount of $$, or all the $$ in the world are not worth a loaf of bread.
Look up the currency museum (I forget which).. But the point is that ever since humans have moved from real money, such as gold (still valuable), to fiat currencies, ALL, thousands of fiat currencies in history have ALL become absolutely worthless with no exceptions other than collector value.