It sounds like the storyline where “Han dies, Leia becomes queen, Luke wanders off” was Kurtz trying to find an ending to ROTJ that wrapped up the series, but matched the darker tone of Lucas’ previous ideas from the ESB era for the third SW film (when it was supposed to feed into the ST).

Not to mention it would also allow an opportunity to return to the Sequel Trilogy at some point. Luke would basically be the same lone wanderer as in the earlier Sequel Trilogy conception, and Han and Leia would both be out of the picture, which also fits the earlier plan.

But it seems Lucas wanted to tie up what was then supposed to be the final film with a definitely happy ending. Particularly one that resolved the Han/Luke/Leia love triangle in a less dour way. Han sacrificing his life for the Rebellion was all well and good as a story idea back when Leia could end up with Luke instead, but ESB basically killed that idea (no doubt drawing on Carrie Fisher & Harrison Ford’s real-life intimacy). So Lucas kept him alive.