How long do you think will the cryptocurrency be popular? Does she substitution the usual fiat money? How many years will pass before this?
It will be popular for at least a long time, but as a maximum - always). Fiat money, most likely, will not replace. But there are already online shops where you can pay with crypto currency.
Yes, they will just be able to take a place next to paper money, as on the Internet and possibly in other structures)