Well, currency will always be worth less than the money it's backed by, simply because it's a claim on the money, not the money itself.

Moreover, currency is commonly issued via lending, which means interest will also be required. If I lend you the first $100 and charge you 5% per year, for 5 years, where will you get the extra $25 in interest to pay me back? You'll need to take out a new loan to pay back the old loan + interest, and so on.