It is true, BTC is very unstable in its price, this is one of the defining characteristics of decentralized cryptocurrencies, however, this volatility is compensated by the fact that in the long term these currencies revalue, something that "reliable" fiat currencies do not do. If 11 years ago someone bought a $40 pizza with 10K BTC what can you buy today with that same amount of cryptocurrencies, i.e. 11 years ago you bought a pizza, in 2021 with that same amount you buy the pizza restaurant with everything and employees. By the way, what is 40$ today?