This proposal of Star Wars being released later on not doing as well is paradoxical. Lets say hypothetically the release of Star Wars was delayed into the 90s, the movie exactly the same as it was in 1977 original release. Same audio mixes everything the same. The hypothetical culture of the 90s sans Star Wars would be different than the culture with Star Wars. Who knows maybe the hollywood landscape would've been much the same in the 90s had Star Wars not made it's initial impact in the 70s. Thus allowing for the same sort of influence only later. Afterall it was the success of Star Wars that made them think about bringing Star Trek back from cancellation(edit or more appropriately in movie form) in the first place. That's just one thing that wouldn't have happened had Star Wars not been released in the time it was.

I would argue it's impact would've been even bigger had it's release been delayed since it seems the execs, and theaters, were straying away from even allowing sci-fi properties a chance. Star Wars was block booked with The Other Side of Midnight to get into many theaters in the first place. Star Trek was cancelled. Maybe if it was delayed into the 90s there'd be so little sci-fi entertainment that Star Wars would've become, almost literally, the only show in town. That sort of impact would've been so much bigger than it's 70's impact. Being the literal only form of "new" sci-fi entertainment after only reruns of old Star Trek or Twilight Zone on the tv.

Course it could've gone another way and some other sci-fi movie could've made it big around the same time. Making a delayed 90s Star Wars release disappear in the haze of that 90s sci-fi market.