Common myth, but it's false. Paper currency's true value is that the issuer is obligated to do something in return for it. Since that's usually governments, paper currency's true value is that you can use it to pay debts to that government. Everybody in the world can stop using US Dollars for anything else, but they'll still be useful for that, because that is basically the definition of a US Dollar. Says so right on the bill.

You can set your philosophical bar of "real value" higher than that, but there's not much light of day between that and a standard so high that it's not even theoretically reachable, at which point I stop caring about your definition of "real value" since when nothing can meet the bar, it's not a useful concept anymore. (But I do mean "not much", not "zero".) In the end, all value stores are transient on a century timescale, and I don't see that changing. Even gold hasn't had anything like a constant value. But for a putative currency, BitCoin is distinctive in its total disconnection from a concrete value.