It is already defined over years therefore it is not easy to redefine money again with bitcoin
Depends on the perspective. Evolution of monies over the period of years was to eradicate flaws the previous form of monies had. Today, we have moved away from the gold standard, ( one of the sound monetary policy ) to fiat. The problem fiat currencies face are that governments or federal reserves are unaccounted for their decisions in increasing the supply. Argentina, Zimbabwe, Chile, are a few examples.
Bitcoin-like systems allow a codified rule to adhere to for the betterment of the system, thereby removing the current problem in fiat currencies provided cryptocurrencies are able to meet other advantages of fiat money.
Some of governments recognize Bitcoin as an asset, not a currency. It is something like a share of a company that can be bought without using a stock market and securely transferred using a strong asymmetric cryptography directly to a buyer which can live anywhere. I think the replacing a fiat money is almost impossible, because the national economies are based on them. The governments will protect the financial system of a country they managed. Yes, the shape of money is changing, the currencies evolve and go forward.
Yes, you are absolutely right. Bitcoin is more of a commodity. And we know from experience, that mankind heavily relied on a commodity as money, animal horns, crocodile money in Malaysia, Whiskey in Germany.
Fiat heavily relies on coercion, ie. pay your taxes in USD or go to jail. A private money competing against fiat money has to rely on persuasion. People who are persuaded enough with the advantage of such private money ( cryptocurrency ) will prefer to hold coins over their national currencies.
On a long run, national currencies and stable cryptocurrencies will be competing based on price stability. Citizens are free to choose one over the other based on evidence of price stability.