Oh, that crazy Emperor, Palpatine...
By any other name, Darth Sidious was one of the most powerful practitioners of the Force in Star Wars history. If you accept my interpretation, he was probably around as long as Yoda, was the apprentice who killed Darth Plagueis (from the scene where Anakin is finally sold on the Dark Side in Revenge of the Sith). So powerful he, ha, feared losing his power...
It's Palpatine who drives all the action in the six film saga, who manipulates the galaxy into a state of war and then recruits Darth Vader to replace him, which leads to Luke Skywalker's bid for redemption, which can only come once the Emperor is finally eliminated. As the Emperor likes to say in Return of the Jedi, these events are all things he has foreseen. He's considered all the variables. His clairvoyance has given him perfect control over everything.
Why, then, does he ultimately lose?
It might be argued that it's a simple case of arrogance and comeuppance, that good must necessarily always triumph over evil no matter how long it takes. It might also be that Palpatine has a fatal flaw he continually overlooked. Call it the Padme/Ewok Paradigm.
People who claim to have thoroughly analyzed all probable outcomes and prepared accordingly often don't seem to consider the intangibles, or in other words the variables of the variables. Sure, you've considered the variables themselves, but these megalomaniacs don't consider that there are ways these variables themselves can wield great influence even if they seem completely insignificant.
We'll begin with Padme Amidala, the queen of Naboo at the start of The Phantom Menace. Palpatine himself is a senator from Naboo at this time, as is the chancellor of the Republic. Since he knows so much about this planet, Palpatine has foreseen the variables even the rest of the planet never considered, that being the reconciliation with the amphibian Gungan race. In fact, he uses Padme's own ideals against her and the resulting weakening of the entire political scene in Naboo at the end of the movie to position himself into his necessary place of power, which he subsequently builds on in Attack of the Clones and Sith to fashion himself Emperor of the Galactic Empire.
As far as he's concerned, Padme is no longer relevant to his concerns except as she relates to Anakin Skywalker, whom he manipulates into becoming his last and greatest apprentice, Darth Vader.
Vader himself represents a flaw in Palpatine's greater design. As the chosen one, Anakin was prophesied to bring balance to the Force, something that hadn't been true probably since the schism that created the Jedi and Sith orders that the Jedi grew dominant over probably in Palpatine's earliest days. Vader was supposed to be the perfect Force practitioner, better than even Yoda or Palpatine. His maiming in the duel with Obi-wan Kenobi was not in the equation, although Palpatine was uniquely qualified to salvage the remains. A Vader who no longer questioned his destiny was exactly what Palpatine wanted, but that didn't turn out to be the case in the long run as it seemed in the short. Vader's son Luke would have been the perfect replacement for him, as far as Palpatine was concerned.
This is only an issue because of Padme, who survives long enough to give the Force a chance to compensate one last time. Anakin does bring balance to it, thanks to Luke, and both are greatly influenced by Padme's refusal to give up the fight apparently lost all the way back in Menace. Where Anakin speaks of frustrations with the system, Padme, who has greater reason than he does to share them, still looks for ways to help it work, even when the system seems completely broken. She forms the necessary relationships that send daughter Leia to one world that can work in the open, and son Luke to a world where things work in secret. In effect she follows the Palpatine model, and ultimately cancels it out.
As the person on the other end of this model, Palpatine was so convinced of its inevitable success that he never considered for a moment that it could work just as well in reverse. Were it not for Padme, the Rebellion would never have happened and the Jedi would have become extinct, the full extent of the Force lost forever. It's Yoda who knows as well as Palpatine the ways of the Dark Side, who can sense instinctually what's going on with the Force. As far as Palpatine is concerned, only the Dark Side is relevant.
If all that sounds a little complicated, consider next the Ewoks. Palpatine was no doubt aware that they existed, but as far as he was concerned they could never have been anywhere near a viable factor in his vision of how the Battle of Endor played out. Even when he's surprised the Emperor always seems to be comfortable that he was right all along, or so his conversations with Vader suggest. When we first meet the Ewoks, it's easy to assume that they're just as primitive and ineffectual as they appear. They certainly have a functional society, but they can't possibly comprehend what all these Rebels and Imperial troops are doing around them. To the Emperor, comprehension is everything. It's probably why he never considered Padme a threat. It's the power of the visionary, who can see the most likely outcome out of every variable, who has thought of everything. Anyone who doesn't function that way can't possibly compete with him.
Except the Ewoks do. They obviously hadn't bothered with any of the other Imperial activities on their moon. It was patently beyond their ken. Yet in concert with the Rebels, they put up a fight anyway, and thwarted the Imperial troops in the end all the same.
One-on-one, neither Padme nor the Ewoks could have done much against Palpatine. That's the real trick. Even the Emperor needed plenty of help, whether it was the Trade Federation or Jango Fett, pawns who played right into his hand, or that insulting fool Jar Jar Binks, who completed the sorry decline of sharp Naboo politics by giving Palpatine the last link of legitimacy he needed to complete his apparent triumph.
The Ewoks are the reverse of Jar Jar Binks. Tellingly, fans don't like either one. Yet one of them is indirectly responsible for everything that fails, and the other for everything that succeeds. It's the paradigm that undoes the Emperor's grand design. His blind spot revealed for all to see, in the end. He assumes only his way, the way he does it, can succeed. And he's defeated because of two different approaches that are the complete opposite. The variable of the variables he considered, the probability of the events he set in motion, is that the reverse of everything he himself was doing could work just as well, and in fact better. Because what the Star Wars films ultimately suggest is that the force of will is not something you can manipulate, but what you are willing to embrace. It's a matter of inclusion rather than exclusion. The Sith work best while hiding most of what they do. Padme and the Ewoks weren't hiding anything (well, except for the whole secret marriage thing). Instead their success came about because they trusted others, even when they had every reason not to.
...And you thought it was all about the Skywalkers...