The cryptocurrencies can be still used as a money but not a day to day type of money. We can still use cryptocurrencies to purchase something in the internet but using it on our daily expenses is not worth it because we will lose money since there is always a transaction fee and you are risking the value of your cryptocurrency with the volatility. Use your cryptocurrency as a money on big purchases and not on cheap ones.
Yes, you can use it but the OP has another argument about it.
As the OP's example :
If you buy a flat with bitcoin and few years later the bitcoin value decrease... Your flat lose his value as well.
Relating your statement to the OPs post. If you are selling Rolls Royce Sweeptails and I bought 2 of it, and as you said buy on big purchases using Bitcoin, and I used my Bitcoins. Let's say one month later and the price of Bitcoin dropped, that also means that the car's value you are selling dropped too, do you get it? We can't use Bitcoin because it is decentralized. Now imagine that continues changes happening every day in our lives, you will never know how much a gum really costs in a day.