So crypto is "useful", since "useful" = "being able to buy things" by your definition
What I would say about cryptocurrency is that it is somewhat useful by this definition, but much less so. With some effort I can pay for food, but it would require a lot of effort to use it to pay for shelter, and it is not legal to use it to pay taxes. Its usefulness is a small subset of the usefulness of fiat currency.
According to parent, "useful" = "being able to buy things". Which crypto lets you do.
So, parent has some thought contradictions
It's easier to barter. By a longshot.
Then, we should say that only "cash fiat" is useful, while fiat bank account balances are not "useful"?
So then your definition of "useful" = "accepted as payment in fiat only establishments". By definition, these establishments will only accept fiat. So you are just defining "useful" = "fiat". Sure, by that definition only fiat is "useful". Definitions like that don't bring any new information.
A better definition is "X is useful if I can convert X into a burrito".
You can do that with DOGE or any crypto, anywhere Visa is accepted (for example with Lemon cash).
You can also do that without Visa, it just requires more work: first exchange your crypto in a P2P network, then pay burrito