>When they were making Star Wars nobody knew if it would be a success

That's to some extent a story Lucasfilm love to taut around "It was the little engine that could!" but its not quite like that. Yeah, it was risky. But it was also a big, fantastical blockbuster, coming on the heels of Jaws, by a star filmmaker whose last film was a huge hit with multiple academy award nominations.

Lucas had talked about two sequels and a possible prequel already in December 1975, and had taken some concrete steps towards making them including signing the actors and some of the crew for two more films in advance.

But yeah, he didn't bother fleshing out the plot for those potential sequels until he got around to making them: Star Wars was made very much one film at a time and as a result is full of inconsitences and retcons.