Personally I do not understand why this is used against bitcoin, a form of money is supposed to be a store of value, the gold that you could use hundreds of years ago to buy something can still be used today to buy a lot of stuff, making gold a fantastic store of value, while in the case of fiat you would be lucky to buy 1% of what you could buy with it 100 years ago, so the fact that people prefer to hold their bitcoin shows how strong bitcoin really is compared to fiat.
Is that really need to be a store of value to be a legit currency? Then we can store values in real state, lands, oil those can be currency too? It totally depends on people. If sellers accept something in exchange for their product then who will prevent them not doing it? Retail businesses and shops don't want to accept bitcoin because of its instability in price. Bitcoin become the ultimate way of storing value but failed to become a currency till now.
People have used a lot of things as money through history, and money has a very specific set of characteristics, like being fungible, durability, portability being a unit of account and being a store of value, the main reason why bitcoin is still not used as a currency has to do with the Gresham's law, if you have the option to use two forms of money, one that losses its value over time and one that gains it, which do you use to buy what you want? And the answer is obvious, you use the form of money which losses value while you keep the one that against it, or in other words you use your fiat while you keep your bitcoin.