From what I understand about what Luke says about the force in TLJ: the Force doesn’t “belong” to any person or group of people (like the Jedi) - it’s not a power that just lets you move rocks and shit. It’s just everything and all the in between in the cosmos - the energy - and you can feel and perhaps harness if you reach out deep enough into it.
It’s not too different from “it’s what gives a Jedi his power. It’s an energy field created by all living things. It surrounds us and penetrates us; it binds the galaxy together.”
What is different, is the idea that it’s always just naturally balanced. The existence of people simply harnessing one side or the other - the light or the dark - doesn’t imbalance it, or the person. Obi-Wan and Yoda believed Vader to be lost to the dark side and evil forever, and that the Jedi and the light side were synonymous. In their eyes, that’s why Luke had to defeat Vader, and restore the Jedi Order. Save the galaxy, bring balance.
Luke has a “new” understanding of the Force, because of his experiences with himself, his father, and his nephew, that the Force isn’t some kind of constantly tipping scale, internally or externally. It wasn’t “too late” for his father to do good, and the inkling that it might have been “too late” for his nephew is just what solidified Kylo Ren’s rise. The dark side is only a cancer because of the philosophy that is and can be. It parallels with how he comes to grips with himself and his failure. It isn’t so different from realizing that his failure doesn’t define him. It’s not binary, and his arcy in this movie just inspires that anyone can be a hero.
I can understand that, but on one side of the scale are Luke’s personal experiences over his six decade life, on the other are a 1,000 generations worth of experiences. Isn’t the real vanity to presume the Jedi had it backwards for 1,000 generations? Shouldn’t Luke just have added another chapter to those ancient Jedi scriptures, rather than to write a new book?