With the law of economics and the ever-decreasing block reward theoretically bitcoin MUST increase in price or else it will fail as a currency. When miners are getting paid less and less BTC the value must go up or else there will be a decrease in miners. Also, when there is a small amount of BTC being created with increased adoption, the price per bitcoin must go up or else there simply will not be enough bitcoin to go around. Some day we will reach the point where either bitcoin fails as a currency, or that units such as uBTC or even satoshi must be used to pay, as some people will not be able to even acquire an amount such as 1 BTC.
With 21 million bitcoin and 7 billion people in the world, that is 0.03 btc per person, and thus that amount must be worth enough to be worth at least the average income in the world which is 10,000$.
Of course not everyone will adopt but if bitcoin is to reach a reasonable goal, something like 1 or 2 billion people owning bitcoin would be ideal.
Well said, I also believe that Bitcoin getting rarer each day because of the lost access to the address by its owner will definitely increase it price. In addition of people adopting Bitcoin, more demand, getting lesser supply, the price value will be an uptrend if we plot this in graph. We then can see Bitcoin beating its ATH from time to time until it become more stable due to the saturation of the market.