I'm not sure that an absence of Star Wars
(and Close Encounters of the Third Kind
) would have meant a complete lack of Star Trek
in the late 70s. What those films did was convince Paramount to back a major motion picture version of the series. If they hadn't have existed, I think a series of telemovies (which were seriously considered as the studio went back and forth between some sort of television presentation and some sort of movie) would have been a possibility.
A bit of a hypothetical tangent, though.