Adoption is important because it brings new people and new capital in the market. With time, bigger adoption, the market will have more people, from investors to speculators and traders, they all contribute to bigger trading volume in the market. With bigger trading volume, big investors have one more reason to join it because they will have less trouble to enter and exit the market with big capital.
Trading and investment is also speculation, because Bitcoin is a speculative asset. The difference is only in time preferences.
And I wasn't talking about market adoption, I was talking about other use case adoption, like payments or store of value. Those other uses are so low that they don't have influence on the price.
I agree about zero-sum game. It is true for traditional markets like stock market too.
Stock market is not a zero sum, because companies manufacture goods and provide services - they create something valuable for society and a lot of people benefit from it. So with stocks you can get dividends, but with Bitcoin you can only get profit if someone buys it.