The law of supply and demand. Since bitcoin has a supply limit and mining difficulty, no doubt that its value due to its scarcity will surely increase. We just have to wait for that moment to come and enjoy our profits.
Sure there is that and I dont disagree exactly but also Im a cynic and dont believe in simple outcomes. I would just say theres a feedback effect so people are aware of that final destination and to some extent its already accounted for within the current holding of BTC.
Believe me there is many holding from at least 2013 onwards just a little bit but enough over millions of users that it adds up to a fair chunk of BTC being held for this 'event' and this is also something celebrated in every halvening type change to the protocol where the block reward reduces. That very much is a precursor of this final event and helps raise this ratio of supply to demand some.
My final take would be that the block reward does have a purpose of sorts to encourage the mining network. Some of its coming from the price appreciation but also this time when there is no block reward brings a slight dilemma in that we then rely on just transaction fees.
It will be like learning to take off and the end of the runway is a cliff, lets hope the wind supports the wings :p