The way I've always heard it Paramount was impatient to make money off the series by syndicating it. Everyone else, including CBS wanted it to return. The reruns on ABC were irrelevant to the decision to cancel.
WKRP in Cincinnati - after a somewhat disappointing third season (deliberately getting out of the station as often as possible) the series rebounded in the fourth season with an explosive season opener and better episodes (including a revisit to the events of pilot episode) and some continuing...
Unless there were changes in the production staff, the odds are Star Trek would have just gotten worse, with more episodes like "Spock's Brain" than not.