You are wrong. In the show (last chapter 2x11?) Ecorp (which is the world biggest company in that world) launches Ecoin thanks to the two trillion dollars loan borrowed by the chinese gov.
That coin, replaces FIAT money and surpasses btw Bitcoin.
This is the most probably scenario in a future. Banks together launch a new cryptocoin.
1. It did not happen yet (in the show) as far as replacing FIAT. It is what their grandstanding CEO wants.
2. Don't you see a paradox-borrowed two trillion DOLLARS (which implies that they have value, as today) to launch something that would diminish (at least ) $ value. No government would agree to such concoction, especially if $$ are the law of the land as a legal tender.
4. You cannot have real crypto currency as a single country currency. It cannot work this way. In addition, $ is not an only currency in the world, although it is still dominant to a degree (petrodollar, etc.)