When did they leave fiat currency at all if they are using Bitcoin?

Is not cryptocurrency the ultimate fiat currency?

It only has worth because we all "agree" it does via supply and demand for the currency itself, rather than any backing material value. Aside from a bunch of power plants in China, I suppose? But people don't hold Bitcoin at $20,000 because they strongly believe in the material worth of the rural Chinese electric grid.

(post edited to say "fiat currency" rather than the noun "fiat" which means decree or command)