In many respects they are right. Frequently there _are_ greater fools. Eventually someone is left holding the can. The Art world works because there are a few extremely wealthy people who are content to be the 'fool' at the end of the chain. In those cases sentimental attachment or simply being a patron of the arts is enough for them. If everyone expects to make a gain, it's a house of cards.

Cryptocurrencies are a bit different. While there is the same line-goes-up, line-goes-down hunt for greater fools going on, beneath that are people gauging the actual worth. Bitcoin and ETH are vastly overvalued base upon current utility, but if they came to replace money/gold/other they could be worth many times their current value. Investors buying and selling on that basis are not looking for greater fools but are attempting to be more right about the future than average.