Go to any local supermarket and see which currency they are completely comfortable with. It is always cash and will be cash for at least a hundred years in the future. Digital payments have go up but it still uses the same fiat currency in the background - just that the payment method is easier and you would not need the cash physically.
In such situations bitcoin is least known in some countries but might be popular in few, but it is not so in some. Things might change in future, but we only represent a small subset of the world's population.
Hundred years with a paper cash? Are you serious? While they still accept it with groceries and smaller amounts, they won't even accept with larger amounts.
AML laws will prevent all big paper cash purchases in the future, they are already being prevented in some stores. Even banks require a proof of trail for that money when you try to deposit it, and washing that money trough businesses that have accepted paper cash, has been a problem they are now going to deal with.
Even without AML, it's just impractical for stores to hold large amounts of paper cash. It's a security risk, and security for banking that money costs additional money.
Also, when you are talking about things like "it still uses the same fiat currency in the background", i think i need to point out that physical cash is just anonymous representation of fiat money. It's not any more real then money in your bank account. It's same kind of promise of value.