Episode IX completes structural elements of the Skywalker saga that Episode VI did not.
In The Phantom Menace, it is stressed that attachment, even to family, is a path to the dark side. You fear losing what you are attached to:
KI-ADI: Your thoughts dwell on your mother. ANAKIN: I miss her. YODA: Afraid to lose her..I think. ANAKIN: (a little angry) What's that got to do with anything? YODA: Everything. Fear is the path to the dark side... fear leads to anger... anger leads to hate.. hate leads to suffering.
It foreshadows the struggle Anakin will lose over Padme in Revenge of the Sith:
YODA: Premonitions... premonitions... Hmmmm... these visions you have... ANAKIN: They are of pain, suffering, death... YODA: Yourself you speak of, or someone you know? ANAKIN: Someone... YODA: ...close to you? ANAKIN: Yes. YODA: Careful you must be when sensing the future, Anakin. The fear of loss is a path to the dark side. ANAKIN: I won't let these visions come true, Master Yoda. YODA: Death is a natural part of life. Rejoice for those around you who transform into the Force. Mourn them, do not. Miss them, do not. Attachment leads to jealousy. The shadow of greed, that is. ANAKIN: What must I do, Master Yoda? YODA: Train yourself to let go of everything you fear to lose.
Yoda's counsel to Luke in Empire echoes this advice. When his friends are in danger, his best move is to do nothing at all:
LUKE: And sacrifice Han and Leia? YODA: If you honor what they fight for... yes!
The moment when Luke almost turns to the dark side in Return of the Jedi -- when he strikes at his father with rage -- is when his sister is threatened. His attachment is his weakness. He overcomes the temptation when he chooses not to act: he lays down his lightsaber and gives up. And it works -- but only because it spurs his father to sacrifice his life to destroy their enemy and save him. His father's attachment to his family, his fear of losing them, drives him to destroy the hated emperor.
Which means the story arc for episodes 1-6 is in contention with itself. The very thing that triggers Anakin's downfall -- his attachment to his family and his fear of losing them -- brings about his redemption. He doesn't grow, he just fails upwardly. His tragic flaw is that he thinks he can stop bad things from happening to people he loves, and his heroic end is... to stop bad things from happening to people he loves. He never learned any lessons from Yoda. He never really learned any lessons at all, just regretted his mistakes.
Whether Anakin's visions of Padme's danger were true or whether Palpatine sent him false visions, his motivation is to save her from death. Palpatine promises to teach him. With it now established that the Force can be used to transfer life essence, it seems reasonable to imagine that whatever technique existed would have had a similar cost: to save Padme's life, the price would have been another life. The Jedi refusal to teach those arts makes sense. The Jedi were peacekeepers. Imagine the temptation to take the life of the guilty to save the innocent.
Sith: You can save those you love from death by sacrificing those you hate. Jedi: Reject attachment, love everyone unconditionally, so you won't be tempted by the possibility.
This is the tension that is left unresolved after six episodes. Anakin only really rejected attachment as Darth Vader, which is hardly the zenith of his Jedi career or a time when the light waxed for him. His return to the light actually came when he embraced his attachments. They were what motivated him to act. And the act he took was to kill the person he hated most in the entire galaxy.
I argue the sequel trilogy offers a much more satisfying conclusion that resolves this dynamic.
The midpoint of a standard story arc is always the low moment, and in that descent insight is found which informs the final resolution. The Last Jedi gets a lot of hate, Rose in particular gets a lot of hate, and this quote specifically does too, but it's a strong core message that resonates with Episode IX and the overall theme of the entire nonology:
That's how we're gonna win. Not fighting what we hate, saving what we love.
That's exactly what happens. Ben Solo is finally a Skywalker who realizes there are options other than fighting what you hate like a Sith or compassionately detaching and ceding agency like a good Jedi (Anakin and Luke respectively at the end of Return of the Jedi).
Sith: You can save those you love from death by sacrificing those you hate. Jedi: Reject attachment, love everyone unconditionally, so you won't be tempted by the possibility. The Skywalker balance: You can save those you love from death by giving up all attachment to life yourself.
This is the solution which evaded Anakin. The Jedi told him his love was wrong. Palpatine made him think bringing someone back from death required turning to the dark side. In his confusion, he believed them both.