It’s a great point, and something I occasionally thought about myself. I agree, the way it’s presented in anything other than the original trilogy makes little sense.

But, I settled on a ‘head-canon’ explanation of how the rule-of-two could work if you just include the 7 movies and disregard the novels/Johnson nonsense (and one crucial part of Revenge of the Sith which breaks my idea!).

My idea is all to do with how Palpatine gets his massive face wrinkles. In my head, Palpatine’s public face – when he’s a lowly Senator in Episode 1, right through to Supreme Chancellor in Episode 3 – is actually a mask he wears using the Force. When the republic falls, he becomes Emperor, and is confronted by Mace Windu, he is relieved to be able to relax his metaphysical disguise and reveal his true, many-thousand-year-old face.

My thinking was that Palpatine was actually an ancient creature, and has kept himself alive and in the shadows all that time using unnatural powers from the Dark Side. This is why Vader doubts he can defeat The Emperor alone, and how Palpatine has actually outlived all his apprentices for centuries.

But then ROTS showed us that Force lightning gave him the crazy scars and ruined my idea. But I still think mine is better!