That's not really true. In economics-terms, to fulfill the function of a currency, a coin just have to be a 'medium of exchange', 'store of value' and 'unit of account'. The issue with most coins now is that they're not readily exchangeable for daily gds and services, and their value fluctuates too wildly day-to-day. But i think we're getting there slowly.
On staking, if u think abt it, the idea is not too different from u taking ur usd, depositing it in the bank, and earning interest. So is ur usd a currency?