The fact that Bitcoin's supply is fixed means that if there will be an increase in demand, its price will really go up and we have right now experiencing it despite the dip from time to time. What we should note here is that nobody has the power to change the supply of Bitcoin in anyway...today or in the future. In fact, experts are saying that even if there will be 21 million BTC the actually number in circulation can possibly be lower since there are millions of Bitcoin that are already lost and can not be recovered anymore.