Thing is, even if the junior cast members weren't on TV/movies all the time, that doesn't mean they weren't working. Some did live theatre, others wrote, Nichelle Nichols did some work as a recruiter for NASA, to find qualified women/minority people to train as astronauts, Takei took a shot at politics, and so on.
But that IS thre thing: they were actors who could not get lucritive ACTING jobs. So they worked elsewhere (politics can be considered a form of acting, I suppose). Nobody said they were bums or unemployed, but that their acting careers went pretty much nowhere.
William Shatner has never worked as anything but an actor his entire life. According to his biography, anyway.