No one buys stocks for the hell of it, it is 100% a financial instrument used to profit by the buyer and the seller. If you couldn't sell to someone for a higher price, no one would buy them. If that's your bar, then cryptocurrencies are as legit as stocks as far as a financial tool is concerned. Stocks may arguably have more utility, but for the buyers/sellers, they aren't much more that a speculation instrument.

If someone will take Pokemon cards for a pizza, or a house, whatever, then it has some utility. If someone collects pokemon cards, and has enough for a house, they might (probably) need to exchange them for some real currency to complete that transaction. But pokemon cards are not inherently a scam.. just because the price can go up and down.. they are probably considered a collectable.

Now cryptocurrencies are not collectibles, and they are not currencies, but they have attributes of both I'd propose. They are entirely speculatory in nature, but can sometimes be used as a currency. So, in some ways, I argue, they should be treated like pokemon cards, in other ways, regarding regulation, they might need to be considered a currency to be able to regulate the industry in such a way that protects investors, just like private transactions (selling collectibles) and financial investing (trading stocks/etc) have regulations.

Although this is nuanced, and cryptocurrency is not currency or a collectible entirely, it is not inherently a 'greaters fools game' any more than trading stocks or Pokemon cards.

It's just another thing that needs to be regulated, and if people didn't use it for something, it would just go away, and we wouldn't be having this discussion.