A currency has value because you can force any shop keeper to accept it as payment against some good, and in certain cases you can convert it against gold. The minute commerce doesn't accept the currency for payments and the central bank won't convert it for gold, it has become worthless, irrespective of how hard it is to make copies of the bank note.

And keeping in mind very few people hold much currency. Usually you hold a deposit account, i.e. you lent to your bank. It is a low risk/low yield investment.