The best argument against cryptocurrencies is simple: while they might be a hedge against inflation / currency crises, they are a terrible one. You can't seriously claim that it is a sound insurance policy to buy an asset that can randomly lose 90% of value for reasons completely unrelated to the condition of the economy (as most major cryptocurrencies did in 2018). And where similarly violent swings in the other direction happen for reasons unrelated to the health of national currencies, too.

You have plenty of other choices. I mean, here's a pure form of insurance: buy far-OTM commodity or foreign currency futures options. They easily pay 100-to-1 if a collapse happens; otherwise, they expire worthless after a year or two. In essence, you can buy insurance for your net worth for 1% of the value of your assets, which seems like a pretty reasonable price.

In contrast... how does that cryptocurrency insurance does even look like? How much of your money do you need to put in Bitcoin to be insured against crashes? What happens with that money if a crash doesn't come? Is Bitcoin guaranteed to keep going up no matter what? If you think you have answers to these questions... you're wrong. You might get lucky, but you're still wrong.

Cryptocurrencies are exciting. But they are speculative. Nobody knows what's their staying power, which cryptocurrency might win, etc.