Rian just had this silly Zoroastrian-inspired idea of darkness rising to balance out the light, and vice-versa, perhaps the result of a corrupted interpretation of Lucas’ vague nonsense about balance in the Prequels. It sounds like some ad hoc idea Rian invented to justify Luke giving up on the Jedi.
This take on the Force is rejected by the movie. It’s a [popular] expectation (gray Jedi, anyone?), in the same vein as EU Luke, that is disposed of to reinforce the Original Trilogy. This where it gets so messy in reception, because Rian’s engagement with Star Wars, like everyone’s, is personal and varied and doesn’t fit into a box.
He doesn’t do an idealized, super Luke because like me he saw that Luke literally didn’t beat the Emperor with his powers, he bet on his dad and his friends. The type of guy who literally did take himself out of a picture so that he wouldn’t endanger the mission on Endor. That’s the interpretation. You don’t have to agree with it or how it was done, but it emphasizes Luke for who he was, not as a trained Jedi, but a son. A farmboy in over his head, just a guy, like you or me. That’s why he resonated [to Rian, to me].
That doesn’t mean he was a “lie”, and it all has so so very little to do with the prequels, or the Jedi as an institution or even an idea. This is a trilogy bereft of any of that kind of worldbuilding or connection - we all know it - but all of a sudden that has valence in this particular critique? No, it’s a personal character arc: Luke embracing his flaws and the triumph he is capable of even with them. It’s more analogous to impostor syndrome than it is about history.
In what way is it rejected by the movie? Both Luke and Snoke tell you explicitly that’s what’s going on, and there’s a big yin yang symbol at the Jedi temple. One of the only scenes Rey has where Luke is actually teaching something is when she’s meditating and sensing all the opposites on the island: hot/cold, life/death, etc. and light side/dark side is one of those, then “balance.” You can infer that the evil mirror cave is on the island because the Jedi there wanted balance or it came to exist because the temple was there.
You’re describing exactly why Luke is the ideal and why people like him. He already overcame himself and conquered temptation. This reverses that development. It would be satisfying to see that inner growth manifest as outward physical power, sure, because that would be cool. But at the very least by basic storytelling logic he should be the wise mentor figure here who has something to teach the next generation. You’re going to say that he was. He was not. By the text of what actually happens in the movie, he was an unnecessary waste of time and effort and the big lesson he learns at the end was that he already blew his chance to teach about “failure”.
It has everything to do with the prequels. Everything Luke says is because he “watched the prequels” as it were.