What you are saying is that currency is only then a proper currency, when exchange rates are "within what I like %", or even fixed, right ?
This was possible, when currencies were bound to gold in the 'old' days.
Now, with fractional reserve economy, currencies freely float.
And maybe you like 5%, some other likes 1 % and someone else likes 0.5% fluctuation as the 'magic allowed fluctuation'.
Look at this chart and tell me USD is a stable currency....
https://www.exchangecurrency.com/wp-content/uploads/2012/05/us-dollar-devaluation.gif