I didn't say that fear was Palpatine's Achille's Heel."Fear ultimately proved to be Palpatine's achilles' heel as it caused him to constantly plot and devise schemes against his enemies."
But that's what said the link from the Star Wars Encyclopedia that I posted. Which proves that you are wrong.
That describes the 8's deterioration to 5.
"They [8s] think that they will be able to maintain their power by being shrewder and more premeditated. Instead of acting recklessly, they will attempt to act with more foresight [especially using the Force to see into the future]; by being more secretive, they will be able to strike without warning [such as the result of Windu's attempt at arresting Palpatine]; by being more wily, they will be able to hide from their enemies until they are ready to defeat them utterly [Palpatine spent years hiding his abilities as a Sith Lord until the moment was ripe]. In short, the temptation for Eights to go to Five lies in the union of power and safety, a seemingly unbeatable combination [and it was unbeatable for becoming Emperor]."