I think we can all agree that TNG was a success too. It was one of the most successful first run syndicated shows.
But TOS was a success too. Unfortunately for its fans its true success was realized after the show was cancelled and it only got 3 seasons. But how many cancelled series can you name that got 6 movies and started an entire franchise?
And heck it wasn't just unfortunate for the fans. The actors were suddenly part of this big hit series but they weren't getting paid for it.
I am not sure what time slot TOS was on its first season but in its 3rd season it was pushed at 10:00 Friday night. One of its main audiences was college students, what do you think they were doing Friday night (and remember this was well before VCRs)
When the reruns went into syndication & could be put on better times it proved it had a size able audience.
I actually think Star Trek's success story is pretty historically interesting.