It's very well known he wrote a basic story and realized he couldn't make the whole thing. He broke it into 3 parts. There's a reason it all started with ep4. That's absolutely a plan. It was enough of a plan that he knew that there was a ep1-2-3 and ep7-8-9. All he had for those is rough story points but that's still a plan. Something he knew he could come back to. Again he had no idea he would even get a 2nd movie. Hence it was just star wars when it released.
This is not what happened and is part of the grand plan myth.
It started with just what became
Star Wars. It was not Episode IV at the time of its writing or release. Various drafts of various names had a bunch of extra backstory and exposition, some of which would survive. The first big idea of multiple films that Lucas talked about was an anthology series of about twelve different movies. He wanted to get other friends and film makers like Francis Ford Coppola to make their own movies set in the universe. He himself would maybe do a single direct sequel or single prequel about Obi-Wan, Anakin Skywalker
and Darth Vader.
The first sequel to
Star Wars was the novel
Splinter of the Mind’s Eye written by Alan Dean Foster. It was written with the idea that it could cheaply be produced as a sequel and, as would be the case with the Expanded Universe it would launch, featured very minimal input from Lucas. In a post-
Jedi world it is notable for the very clear romantic elements between Luke and Leia because them being siblings was Lucas trying to come up with another twist to insert into
Return of the Jedi following the success of turning Darth Vader into Anakin Skywalker.
Lucas had some ideas for the sequel, but hired Leigh Brackett to actually make a cogent narrative and take them somewhere. Early drafts of
Star Wars Chapter II included Luke having a sister (with Lucas starting to toy with it being Leia but not yet committing) and a Force ghost visit from Anakin Skywalker. Obi-Wan talks about Anakin as a separate person because he was a separate person, something supported by early drafts of the sequel and Lucas’ own musings for a single prequel.
The idea of the original trilogy being the middle of something came during the development of
The Empire Strikes Back.
Star Wars didn’t even get retitled until 1981, the year after
Empire was released sporting Episode V in its opening crawl. Lucas’ story of what this meant would change over the years. Sometimes it was just world building to make it bigger, sometimes it was the “trilogy of trilogies” and then for awhile it was just the second half of what he insisted had always been the six part story of Anakin Skywalker (even though he had very clearly been a separate character from Darth Vader).
Recognizing the facts of history and even the precarious notion of appointing a new Lucas does not in any way excuse how Disney handled the property. Knowing context and understanding motivations is not a validation of process and decisions. More importantly for these types of discussions is it can help you realize who is absolutely, completely full of it and should not be taken as a reliable source of information regarding what went wrong.