The wonderful Chuck Klosterman turns out to be a computer games fanatic. In an interview about this piece on games' analysis, he asks
"So when does something become a critical element of the zeitgeist? It's when people who barely care are interested. It's when you can move beyond the person who is actively involved in the game."
That seems as good a definition as any of the difference between niche and mainstream acceptance and the crux of the matter is whether you want to deal with customers who "barely care".