The problem is that money can become something like another fashionable product. Why using Bitcoins when you can select another system or currency? Like we choose between Whatsapp or Line or any new product that appears on the market.

Moreover, supply of Bitcoins is limited in time, ok, but when the limit is reached and the currency is not useful to buy low price products... What if people create another digital currency like bitcoins but no bitcoins, "Low-Bitcoins"? People could use this new currency (lower in value), and external to Bitcoin top-limit system, to buy these low cost products more easily than with Bitcoins. When this happens, the money supply will be increased.