hi, I think they are a fantasy, but I’m aware that “real” money is also pretty much a fantasy, apart from some gold bars that we have to assume are existing somewhere.
But to produce (mine) BTC and ETH (and at the same time verify things) costs way too much energy, although I know there is controversity here. But
Imagine they would be used at the scale of real money currently …
However, you’re right that the insides of their mining / verifying mechanism are quite a mystery to me, and that I may miss the point as for IOTA. I just had the perception: IOTA = no mining = no electricity waste.