If/when fiat collapses, then crypto will likely take on an additional characteristic of money, becoming a "unit of account". This means that goods and services will then be priced in crypto just like they are now proved in dollars or yen or...whatever. So, the value of crypto won't then be determined by how many dollars or yen it can be converted into but rather by how many Big Macs or Starbucks coffees or Tesla cars one can buy with it.