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Fiat also has no guarantee to always be stable because fiat can also be eroded in value by inflation and also by a sharp increase in the price of goods. And from several assets that I often monitor for their stability until now is gold because gold is still relied on by some people as a store of value that is so stable even though there are always increases and decreases in its price in a small percentage. As for Bitcoin, it is more of a future asset that can be relied on to be able to have sufficient profits after we collect it slowly through purchases.
No currency is stable - its price changes all the time. Bitcoin, fiat, gold and altcoins are all volatile - but their volatility varies. If you want to hold bitcoin for the long term, then by all means hold it, but also hold other currencies like fiat or perhaps gold. Bitcoin tends to be seen as a future investment asset and some people see it as a hedge - it has very good returns in the long term, whereas gold and fiat do not perform that way.
I'm not sure you and most other people can transact without fiat in countries where bitcoin or crypto is not legal as a means of payment. Fiat is needed in such cases - but bitcoin can still be used secretly, especially if you are willing to do so. But for the long term - I think bitcoin is the most recommended compared to gold or deposits.