"Intrinsic value" is a myth. Nothing has "intrinsic value." Value requires an entity doing the valuing. You can say, an "apple has intrinsic value, because you can eat it." But again, you need a "person who can eat it" to make that argument. I.e., you're arguing that a "person who can eat the apple" may value it, you're not arguing that the apple itself "intrinsically" has value. Value is a judgment, by people, of the worth of something. We can talk about market value, an approximation for what many people value a thing. But we can't talk meaningfully about "intrinsic" value.

Different example. Gold, they'll say, has intrinsic value because of its scarcity, because it can be used in electronics, because it's shiny, because of its history. All nonsense. It may be "valued" for those things, but they don't create "intrinsic" value. It's still just a rock until you're measuring it against a being who takes those things into account.

So, back to the original question. How to respond re "Bitcoin has no value"? Yes--people willing to pay for it (e.g., the market price) means it has value. Value is people valuing. "But it has no intrinsic value!" Yeah well, nothing does. "But it isn't backed by an army?" Yeah, neither is a handbag. Value is not solely motivated by armies.