You are definitely right that they aren't all equal. Generalizing them all is a problem as the outright scams and stable coins are way too different, nevermind varying degrees of centralized control. The only thing they should have in common is a distributed ledger...then there are the privacy coins which anonymize those. In fact a crypto just implies cryptography, so is distributed ledger even necessary?

Stable coins are a good example of crypto that isn't volitile. Baskets of stable coins (ie. Libra) may be more stable than fiat.

So I think it is best just to stick with BTC and maybe a basket of the top 10~30 by market cap....let's call that a crypto etf.

BTC cannot be taken away, but it also cannot be given back when it is gone. Could you imagine the pressure if a country lost 100000 btc to oblivion because of a mistake or virus? They can easily reissue fiat. Also most people will give up BTC or at the very least take it to the grave when threatened with violence. So it can be easily erased against your will.

Interesting implications. The reason it hasn't been an issue yet is because it's value compared to the overall economily is low. With mass adoption these problems may present themselves. Nuke the hardware wallets and the crypto private keys are gone forever making yours that much more vauable.