I'm just reading an article in the newspaper. Klaus W. Wellershoff (a last chief in UBS Bank) says that's impossible to use a currency with volatility as money. If the value of the money decrease, that means the value of the merchandise decrease. For example : if you buy a flat with bitcoin and few years later the bitcoin value decrease... Your flat lose his value as well.
What do you think about this argument?
This sounds pretty nonsensical. Real estate is an asset class in itself. How does it matter if you bought it with bitcoin or something else. The value of the flat will depend on the overall outlook of the real estate market.
It would be better if you can provide a link to the article because it seems like you are misreading something.
Although the argument that a cryptocurrency with too much volatility cannot be used as money has its merits but the volatility seems to matter only because we are comparing the value of bitcoin against fiat. In a scenario where bitcoin is actually working as currency, there will be need for a set of indices standardized by calculating amount of common goods that, say, 1 BTC should buy. Such an exercise would either need regulatory intervention or bitcoin adoption would have to go widespread with merchants deciding this value governed by free-market competition.
This is when volatility would not really matter.