If the value of the money decrease, that means the value of the merchandise decrease.
I think it's the reverse--if the value of, say, fiat decreases the price of good goes up. I'm thinking of hyperinflation situations in particular, where the price of stuff can go up crazy amounts. You've seen those wheel barrels with Zimbabwe dollars in them, right? Well yesterday it cost 100 dollars to buy X, whereas today it costs 10,000 dollars for X. The value of the money went down quite a bit, and as a result the price of X is high.
The example of buying a flat I think is correct--if the flat was valued in bitcoin only.
As far as volatility being an impediment to bitcoin being a currency, I agree with that as well. When it's going up, that encourages hoarding of it, and when it goes down you can't spend it fast enough. You can still use it as money, of course, but the volatility is definitely a negative attribute as far as that's concerned. But that also makes it great for trading.
That's true, and the post below also made the valid point of the price (hopefully) normalising later on. This is more applicable to when a particular cryptocurrency gains mass adoption so the actual usage is the main driver behind price, instead of speculation.
When you think of shares, the more established a company becomes, the more stable is the price.
However like shares and fiat (FX markets), cryptocurrencies will continue to have some traders that affect the price through speculation. For example when a large company announces a new venture in a new country, the price increases be a decent %. Or when a country gets downgraded in terms of its credit rating, the price of that local currency/fiat depreciates (falls), as less people demand it. This doesn't make that fiat currency be inappropriate for use as a currency or money.
One can also think back to the old days where people would use other things as money, such as wheat or dates. In such cases, the price would be affected the demand and supply of wheat, leading to sometimes large changes in the price. However they remained being used as money for centuries.
The issue with fiat is that authorities can print lots of fiat almost at will, which can lead to sharp changes in price, even in the short run. This cause doesn't occur with bitcoin.