Gold was no longer a guarantee of banknotes long ago. The current fiat currency guarantee is government credit. The more credit a government has, the more stable the price of its banknotes. Positive examples are the US dollar, the euro and the Japanese yen, while negative examples are Zimbabwe, Venezuela, and most recently Russia.
Similarly, bitcoin's guarantee is a consensus that bitcoin proponents affirm the value of bitcoin. From my personal point of view, this value is a relative value, and the counterpart is fiat currency. How much do you not believe fiat currency, to some extent you have more believe in the value of bitcoin.
because money is not as complicated as bitcoin that functions as a commodity like gold and property, in the future bitcoin even has the ability to become a fiat currency guarantor with various altcoin in it (national and legal). If bitcoin is applied today it is very risky because it will bring active inflation. so bitcoin should stay focused on its own market and go hand in hand with other economies as a benchmark.