If you invest in a company you can be investing in a productive asset that will produce more than you put in. You get a cut, but society does as well, and everyone is better off. With crypto, you're getting your money from the person you offload the crypto to. You're richer, they're poorer. They can try to offload it to someone else, but eventually someone will be left holding the bag. Things that can't go on forever, won't.
Even in the highly unlikely best case scenario where these assets stabilize at high prices, you're expanding the monetary base and getting your money by inflicting price inflation on the rest of society (the amount of products isn't increased, but the amount of currency trying to get those products will be).
If they are marketed as temporary, volatile, risky currencies, then sure, they are not a scam.
The whole crypto industry reeks of sensationalism and trapping gullible idiots who are fed dreams of "returns".
What would make a market melt up illegitimate? Traditionally I was taught that pump and dump and painting the tape are illegitimate market behaviors. Otherwise is a mania illegitimate or isn’t it just a natural market phenomenon?