>If I sell stock through my stockbroker, I can have funds in a bank account within hours.
Wait, what? I'm pretty sure you're exaggerating there. Stocks have a two day settlement period. (I know because that knowledge gets firehosed every time Robinhood/GME comes up.)
Last year when I sold stock in one account (Wealthfront), for the proceeds to deposited into another, it took four business days (edit: using ACH). When I complained on social media, my finance friends said that was typical. Now, it might have been faster with a wire, but it's not the hours you talk about.
>That's not a limitation of USD, it's a limitation of sleazy crypto exchanges.
It would still be an issue if you want to convert to USD purely on the blockchain because you're interacting with multiple smartcontracts. The USD would need to be a cryptocurrency that lives there.