I think what you’re getting at is that the publicity generated by flashy boycott activism only generates free advertising for the companies. Which it certainly can! But that’s also dependent on what is being boycotted and the social and political beliefs behind it. If one group boycotts a product because the company is homophobic, another group buys more of that product because they agree with the company. That sort of thing.
But it isn’t as two dimensional as “boycotting has the opposite effect”. Here are some examples of effective boycotting. Though you did get me interested in how effective boycotting really is, but I couldn’t find any efficacy studies that weren’t behind a paywall…