Something I would like to add. I think that some may be confusing what constitutes a religion going "mainstream".
However, far too many people attach the label of cult to something that the do not like, a factor that diminishes the usefulness of the term
James Penton uses the terms "sect" to describe religious movements that are insular and not integrated into society at large. That term still carries with it negative connotations, but not nearly to the degree that the term "cult" does. Cult is a pejorative that is not very useful to throw around, in my opinion. No one would admit to being an "idiot" more than anyone would admit to being a member of a cult.
What's interesting, as Penton points out, is how long the JWs have resisted integration into society at large. If you look at the LDS, which started more or less around the same time as the JWs, they are far more integrated at this point in their history. They sponsor hospitals and universities. They have governors, senators, and even presidential candidates among their members. By contrast, the JWs still largely operate outside of society. Few Witnesses leave a lasting imprint on society at large. Very few lawyers, doctors, artists, public figures, etc. among their members. The revolving-door nature of JWs whereby its membership is almost completely renewed every few decades is probably behind this.