If they aren't, then the price remains unchanged, advertisers drop out of the market for BAT, and fewer ads are served. Again, you reach equilibrium, except that people who do not feel the current price of BAT is worth their attention don't have to put up with ads, marginal advertisers who get little value out of the advertising don't run them, and Brave makes less money. This also creates incentives on Brave to ensure that their advertisements are targeted effectively (so advertisers are willing to pay more), that they're unobtrusive (so users are willing to put up with more), and that the market for BAT is efficient (so value isn't lost in transaction costs).

Markets and negative feedback mechanisms are extremely powerful things. Most of the things wrong with the modern corporate economy can be traced back to markets being replaced by hierarchical organizations with positive feedback mechanisms (eg. corporations having more money with which to buy up competitors, which leads to increased pricing power, which leads to more money to buy up more competitors; or increased lobbying spending leading to favorable regulations, which keeps competitors out, which raises prices, which leads to more money for lobbyists).