The original "Star Trek" TV series was marked for cancellation early in its run on NBC, because it didn't do so well in the ratings. But the fan passion... now THAT was another matter entirely, and still is.
Star Trek lived for another season, then met its demise. Its first one, anyway. Now the Paramount franchise totals 12 TV series and 13 feature films over nearly 60 years, largely because it has had a knack for reinvention when change was called for.
-The history and the personalities are recounted in Ryan Britt's book Phasers on Stun! How the Making (and Remaking) of Star Trek Changed the World.
The author interviewed more than 100 actors and writers from the various Star Trek entities; he sits for an interview on what he learned.