At which point your local grocery store or gas station wouldn’t be accepting whatever currency is your current local currency. The point would remain the same - a currency doesn’t have to be universally accepted everywhere on the entire planet for it to still be a useful currency.
Does my grocery store or gas station accept it?
Just because it’s useless to you doesn’t mean it’s useless in general.
Does your grocery store or gas station accept Qatari riyals?
If that were my local currency, then I’m sure they would
At which point your local grocery store or gas station wouldn’t be accepting whatever currency is your current local currency. The point would remain the same - a currency doesn’t have to be universally accepted everywhere on the entire planet for it to still be a useful currency.