Trek was referred to as a "franchise" or the "franchise" before DS9 or Voyager. It easily could have been called that before TNG's run (which I believe is true).
At the time of the late 70's to early eighties Trek was huge on national daily stripping, and since actors, writers, directors only got residuals for a handful of broadcasts it was all pure profit for Paramount.
You even had studio people refer to Trek has a being money printer. All profit at no costs, besides the initial purchase from Desilu.
Then teh film franchise started, and while TOS films were never huge, they were very solid dependable performers and until new Trek arrived each was in the top ten of the year it was released.
During those two decades no other property was successful in both sides of the visual entertainment industry.
Of course when TNG arrived, in steadily grew to becoem a huge national program, generated millions, but it also hit the profitability of TOS reruns (which took a sizable ratings hit and never recovered), and the Trek films never again when new Trek aired performed strong enough to be a top ten performer.
And as crazy as it sounds TNG at least in teh first couple years probably hit TOS reruns hard enough that the tv side of Trek probably made less money then before TNG aired. again TOS cost nothing to make tens of millions every year. While at first TNG, wasn't a huge as it it become, and at was an expensive program at the time.