Kai Bamarri
It's important to remember a few things.
We throw around terms such as "Balance", "Will of the Force", "Light Side", "Dark Side", etc., which don't actually exist. They are man-made ideas by characters inside the Star Wars universe, to contextualize their experiences with the Force. In much the same way that time is a man-made concept and doesn't exist within our own universe. It's merely how we contextualize our experiences and events here on Earth.
All this to say, characters may say and do things with some measure of authority within Star Wars, but they are all smaller parts of a larger whole, with their own incomplete, and oftentimes imperfect, perspectives. No one character's words or actions can be taken as definitive truth, though some may be more true than others.
What we do have, however, are the words of the creator, George Lucas. The definitive source of canon in Star Wars. And his words on this subject cannot be interpreted any other way. The Dark and the Light cannot coexist, and the prophecy of the Chosen One was fulfilled by Anakin Skywalker.
Simply put, what fans and characters call the Dark Side is the inclination to exert one's individual will upon oneself and the galaxy, free from collective restraint. Conversely, the Light Side can be said to be the collective will of life itself, in perfect equilibrium. The Force is a powerful tool; in the hands of the wrong (Force-sensitive) individual seeking power it is capable of devastating destruction and risks becoming a threat to all life. Therefore the Force, as an avatar of life itself, cannot allow itself to be corrupted like that, and will work to destroy this inclination wherever it manifests, whether around an individual or a group.
The Force is a pathway to many amazing abilities, but power always comes at a cost. No matter which "side" of the Force you are on. You must sacrifice something.
Because you are ultimately not the one in control.
Characters talk about how balance must be brought to the Force, which you identify as the eradication of the "Dark Side", yet the galaxy remains in a constant state of flux and evil is never completely eradicated, even if all the current Sith in a generation are killed off.
Yes.
No one said the so-called "Will of the Force" was fair, or just, or morally "good"... except Jedi. This is why writers are so in denial and try so desperately to rework the true nature of Star Wars into a philosophy more acceptable and pleasant to our own human conditions here, in real life, rather than the horror setting Star Wars really is. But in the end, these efforts amount to nothing more than wish fulfillment. There is no end, there is no escape. The wars will continue indefinitely, from this age to the next, and untold billions will die, as the Force continually rebalances itself.
Once you've realized that the Force is a demiurgic entity, and that neither the Jedi nor the Sith offer an adequate answer to this problem, you've begun to take the Kreia pill.