neither supply nor demand of anything are predictable and stable why should prices be?
prices need to reflect the change in supply and demand, thats why any attempt to make prices stable is nothing but a distortion of the market and the will of consumers.

its exactly because bitcoin's supply is completely predictable and not dynamic that its such a good form of money, things priced in bitcoin reflect exactly the balance between supply and demand without anyone manipulating it by increasing or decreasing the amount of money.