The average "boycott" is not even organized, has no particular goal, and is just meant to make the boycotter feel morally superior. I bet Starbucks execs giggle every time they hear about the next boycott. The people calling for the boycott probably haven't been in a Starbucks in 15 years, and they don't have any intention of becoming regular customers if they do.

Thinking of boycotts that have worked in the past, they generally had specific demands and a plan to resume normal consumption when those demands were met. The Gallo wine boycott in the 70s was successful. The workers had a clear case, simple demands, a desire to negotiate, and a call to boycott one specific winery until they came to the table. When they did, the boycott was lifted and the majority of boycotters went back to consuming the wine.

On the other hand, if I decided to boycott Gallo wine, they wouldn't notice, because I don't think I've bought a bottle of Gallo in my life, and I haven't given a good reason to do it for other people to join me.

Just a note that my "Starbucks boycott" thing refers to the people over the years who have boycotted Starbucks for random greivances, not the organized union boycott, that might have some success.