I think what is forgotten is the context of the times. I'm not suggesting the folks behind the film timed its release intentionally. What I am suggesting is that society was ready for something new and they didnt even know it; nor were they prepared for it.
Star Wars makes you feel good inside. It's corny sounding but its true. It is, unlike nearly all other film since or before, like a beacon of hope. It came at a time, when people needed it. Art is a reflection of life and if you look at the films of the time, edgy and dark were standards.
I would also add that I'm not asserting that anything today, even another great Star Wars movie, could have the same deep cultural impact. Afterall, part of that feeling of hope comes from the fact the film presented technology that hand't existed. But a good Star Wars film released today could stand apart from all other films; even those of the same genre.
Cinema today is obsessed with darkness. I like the Dark Knight as much as anyone but it is representative of the dark, moody, and edgy style of film inundating theaters. And many of them try too hard to be clever. Plot twists are almost pre-requisite and most are poorly done. There are light hearted movies but they tend to be either character driven or lack any real substance.
Above all, Star Wars was an escape in a time where people wanted to escape. When I first saw Star Wars as a kid, for those couple hours, I was in a completely different world and I was stuck there for about a week after and maybe even longer. Who wanted reality when you could lose yourself in that universe? When was the last time you saw a movie where you could totally lose yourself or your sense of time or reality for a couple hours? Granted I was just a kid and naive but how many adults at the time were absolutely taken by it?
And I dont dismiss the fact that the product itself was outstanding. I judged all other movies as 'other movies'. Star Wars was just a different category.
That's just my opinion. Technology has caught up with our imagination so it would be difficult for any movie to do what Star Wars did. Maybe I'm just hoping that the current cinematic landscape of cookie cutter films is ripe for the taking. Maybe it wont even be a Star Wars movie that impacts our collective conscience next.