I’m not entirely convinced, although I’m partway there. When you “have” a Bitcoin, that means you have the private key to a wallet which the blockchain says has such-and-such amount in it. You can have that even if the miners all disappear. You can still use it to sign transactions, although they won’t get confirmed. You can even confirm them by mining a new block yourself with your transactions in it. (If all the miners are gone, the difficulty has hopefully dropped precipitously. And if not, you could always fork it to have a lower difficulty, since the only thing that stops forks currently is network consensus.)

This doesn’t sound super useful, but it sounds about as useful as a bar of gold in a world where nobody considers gold to be valuable.