Yeah, but look at it in terms of the time it was released. Few, if any, sequels had ever come out by 1980. Most considered the success of ANH to be a fluke anyway so they never really gave Empire a chance. And the ending of Empire was such a downer that most "non-fanatic" fans were unlikely to see it without knowing that the story line would ultimately be resolved. Imagine if you had seen Empire, but very few others did making it a horribly attended movie. Then, due to the lackluster BO, Jedi was never made. You'd be left hanging and I think some people may have been afraid of that possibility after hearing from reviews or from friends that there was some big, depressing cliffhanger at the end. In retrospect, it's brilliant.

As you say, it's not your average sequel. But did they realize that back in '80?

Just possibilities.