Most cryptocurrencies are cartoon characters particularly tailored for gambling machines. I haven't seen a single use case other than enabling masses enter a massive pyramid scheme constantly manipulated in social media.
that the reason someone might pay you more in the future must be "legitimate" - that is the asset can be independently valued to be worth more in the future.
Investment in shares is not a scam (mostly), even if they _never_ pay a dividend, as long as they earn a profit. Investment in gold, however, can be likened to a scam, as the value of gold only grows to as speculation, and not wealth creation.
Hard to say whether cryptos behaves as gold, as some crypto does have uses beyond being sold to someone else for a higher price.
Well, gold has numerous uses in the real world. https://geology.com/minerals/gold/uses-of-gold.shtml