I do agree that Star Wars has always been rather generic in its concepts, ideas and world building (Hamill said Lucas once confessed him he took the idea of the Force from hundreds of pulp fiction stories), and I do think that’s partially the reason why the post-RotJ films doesn’t work for me: they have tried to build an entire franchise (with some takes on morality and politics and with more complex stories) from a set of movies that were about an evil masked guy being evil, the good guy being an uncorruptable good guy, and the hot chick being a hot chick, the plot being a mere excuse to watch exciting adventures. You’re probably aware of this, but there was a 1979 paper about Star Wars (I haven’t read it yet, though) where the authors basically said the massive success of the first movie was the result of the lack of any kind of political message or philosophy there: everyone could make a different reading of the movie and insert their own narrative into the movie, which is why many spiritual people interpreted the Jedi religion in Star Wars as being based on their own religions and mythologies. Empire certainly was more scifi-ish than the first movie, so naturally many people uninterested in science fiction never bothered to watch Jedi.
I don’t particulary think the generic nature of the OT is a problem for marketing, because for many 1977 and 1983 people Star Wars meant lazer swords and mind control and a masked dude and Rebels versus Imperials (even the ST has taken a more OT-like line than the boring PT): it’s no surprise the pre-PT videogames took that route instead of being based on the oEU, because after all even in extremely limited systems like DOS or the early 3D consoles you can make space simulators or 2D platforms. In a world where the concept of parallel universes would be part of Star Wars, you could have sub-franchises, each one appealing to fans of every parallel universe, just like now we have PT-era stories that are marketed as part of the “Clone Wars”.
Perhaps the franchise could have taken the original path Lucas had in mind in the last weeks of Star Wars’s production: that every movie could have been unconnected from the rest, set in different eras, with only some aspects that would define them as “Star Warsy” (perhaps the presence of the Empire or the Jedi or “lazer swords”, but that’s it). Sadly, Vader’s massive retcon in Empire, as fantastic as a twist and climax for that movie, set the movies in the Trilogy/Saga direction. I still don’t understand why the main franchise “needs” to be about the Skywalkers (the ST may have a non-Skywalker protagonist but it has Luke and Ben as the two core characters of VIII and IX).