"Return of the Jedi" was kind of a "Mission Accomplished" moment for the Rebellion I think.
I haven't read any expanded universe stuff for The Force Awakens, but it seems like the death of the Emperor and Vader didn't instantly topple the Empire. It was certainly a victory for the Rebellion, but we are talking about a "GALACTIC EMPIRE." They had their feelers everywhere. They still controlled the narrative, there are still millions, probably billions of star systems out there that hadn't heard the news, and were still under Imperial control. I'm sure when they learned that a handful of Rebels, some Ewoks, and a Smuggler took down the Empire, let alone a "Jedi"named Luke Skywalker (I'm sure they were fine with him as the kid who got lucky and blew up the Death Star, but I doubt they thought of him as a Jedi), they were more than just a little incredulous.
It probably took an additional decade of battles and skirmishes to make even a little headway in a galaxy that deep under the Empire's thumb.
A planet like Jakku, an outer rim planet I can see the stories of the Rebel victories on Endor being looked at, and shrugged off in a similar manner to how many Americans shrug off the conflicts and victories of events happening in other countries. There's kind of a dissonance there, but on a galactic scale.
Rey was born quite a few years after the end of Return of the Jedi. She's at least 20, possibly younger. She'd be even further removed from the events before her birth, when, from the looks of things, it hasn't had much of an effect on her life. She's still living on Jakku, scavenging for useful scraps, the Empire still a presence in some capacity, with only the signs of a long ago fought battle any indication that her home is anything more than a dry, dusty sand planet.
I can totally buy the idea that the Jedi, of which there were only three in existence at the time of the Rebellion, and two of them are dead, and one of them hasn't been seen in decades for reasons unknown. I think it's an issue that, sure, is arguable, but not so out of the realm of possibility that it breaks the narrative. 30 years is a long time. I was 6 years old 30 years ago. A lot has changed in the world since then, and this is just one world. We're talking about a galaxy full of billions of worlds like ours, some advanced, some not, that are dealing with events that took place 30 years in their timelines. The further away from the center of the events, the less the ripples will be felt, I think.
tl;dr: The mythical/legendary status of the Jedi is something I can easily suspend disbelief on for this movie.
