At the current stage of development, Fiat is absolutely necessary. Perhaps for the real functioning of the economy, Fiat is not needed, but the consciousness of most people is still very attached to the Fiat. So it was with the first paper money, then with electronic payments.
What do you mean by "real functioning of the economy"? It is generally accepted that any advanced economy, where advanced is anything beyond a simple subsistence economy, money is a must-have, as it were. In a subsistence economy you don't really need money and can get along with barter as it doesn't introduce division of labor other than hunting, gathering, and agriculture. Anything else needs money, and modern economies need fiat as money because money based on hard assets will be ineffective in so complex economies.
The "built-in mechanic" of fiat regulating itself is a complete fantasy. Central banks and governments set interest rates, expand the money supply, established the rules of FRB, create bailouts, maintain liquidity, etc. There is nothing "built-in", nor "automatic".
Of course all of the features you mention can be implemented in cryptocurrency, as well.
You may need to finish school, really. And learn about the forms in which money exists today. Central banks and governments on their own can't "expand the money supply" as it will only cause inflation, price inflation. Hint, you may want to read about endogenous money supply.
Your cryptic remarks are not going to convince anyone. Central banks and governments do "expand the money supply", and it does cause inflation. Are you disputing that? You seem to be claiming that 100% of the money supply is endogenous? Endogenous money supply is a function of fractional reserves and banks making loans. Yes, it was occurring before it was allowed by law, but today it is either allowed or not allowed; courts, eg in the US and UK, have ruled that bank deposits are not bailments.