More people believe that bitcoin has value, more valuable it becomes; that's a simple rule of a store of value. So, bitcoin adoption is in level of people's trust. The higher level of trust bitcoin has, more adopted it is. I think while bitcoin keeps its properties, it will earn people's trust and become more and more valuable. In 20 years, no one will ever doubt that it has value and the exchange rate will reflect it.
Bitcoin as a store of value is pure an added advantages of bitcoin because when you go through the official whitepaper of bitcoin as published by satoshi Nakamoto more than decade ago, the true purpose and value of bitcoin is a decentralized digital currency that give true freedom to people over their financial activities but sadly, majority only cares about bitcoin as a store of value in prder to gain when it experience price increase