Yeah, it is pretty easy, crypto value is built either on Ponzi or on the Greater Fool principle.

National currencies, issued by states are not a scam.

I agree cryptocurrencies need a greater fool to have value. But how then are national currencies different? The same greater fool principle applies: if no greater fool wants the national currency anymore, it ceases to have value.

So crypto is more similar to stocks? Saying they're all scams just because of that seems harsh.

No, stocks are not tied to thin air.

Stocks can crash, and some of them are scams, but most of them are not, contrary to crypto.