This is not so different from any fiat currency. It's only worth something if you believe it's worth something. For that matter, any valuable thing. But at least you can't print gold. (No I'm not a goldbug)
Bitcoin, ethereum, dollars, euro - there isn't anything fundamentally different. Printed money is just that, and the guy who controls the printer can choose to print more and inflate the currency. But we all believe it's worth something... Because we believe it. It's a tautology. Cryptocurrencies are no different.
Except they are different. Gresham's Law--bad money chases out good. We do have a problem with fiat and who controls the issuance, no argument there. However, "money" or "currency" is also based on history. The dollar has a reasonably good history up until 1971 when Nixon took it off the partial gold standard, the value plummeted.
Consider this: Would divers risk their lives to recover paper money from a sunken 1930s era shipwreck? Yet for gold or silver, they will readily do that. I'm not gold bug, but I can understand why--gold and silver have a loooong history as stores of wealth. That's probably more important than any other consideration--how long has the "money" been "money?"
Bitcoin, ethereum, dollars, euro - there isn't anything fundamentally different. Printed money is just that, and the guy who controls the printer can choose to print more and inflate the currency. But we all believe it's worth something... Because we believe it. It's a tautology. Cryptocurrencies are no different.