I think an important Achilles' heel of cryptocurrencies is just how hoarded they are. Whales -- in general, they're just early adopters -- own huge swathes of the total amount of crypto out there. If we were to actually move to a world where crypto is used as currency, we'd have trillionaires who "earned" their wealth simply by being early adopters. Government issued currencies can solve this through taxation or more generally through monetary policy.