When TOS ended, it was not long when NBC (after discovering syndicated TOS actually reached that key demographic when it was first run) wanted the series to return, and as noted 1 post ago, we know what followed.
The success of the original Star Trek
in key demographics has been greatly exaggerated, especially by Roddenberry. It's a myth that NBC only began measuring audience demographics after the series was cancelled. In fact, the fact that the show's limited audience was composed of demographics that the advertisers liked was one of the factors in its renewal for a second and third season in the face of overall low ratings.