If Star Wars had flopped, I'd say that going to movie theaters would have died out within a decade. At the time, Hollywood was bent toward gritty realism. The fantasy Star Wars offered completely turned that around overnight, helping to establish the blockbuster that Jaws started a few years earlier. This was a major contributing factor to the success of Star Wars. It came out at just the right time. It is possible that another fantastic film would have filled the void, but who can say? Nearly all of the fantastic films that were made after Star Wars were made because of Star Wars. If it weren't for Star Wars and its box office, most studios would have been to skeptical of such fare's ability to make any money. As such, public interest in movies would start to wane and movies would gross less and less and eventually it would all but cease to be. Like Drive-ins. There are still a few around. I know of two near where I live, if an hour and a half drive is close. But they aren't as popular as they used to be. Movie theatres would shrink down to those art houses where people would go to see movies on the big screen as a curiosity

And let's not forget the mechandising. Fox simply gave Lucas the merchandising rights, since they had lost so much on Dr Dolittle a few years earlier. Movies has been exploited before with merchandising tie-ins, but I think it's safe to say that Star Wars was merchandised like no film before it.