I think what Chuck fears, above all else, is aging. Not dying necessarily, but aging. Losing relevance, being replaced by the next generation. Which is ironic because he clearly loves the process of creating (he didn’t stop at one world but made many) but he just can’t stomach seeing a new infant creation of his grow beyond the baby stage, beyond the perfect lines they were drawn in.
Maybe the first things he ever created, angels, didn’t originally have free will for that reason. And they all adored him because they had no choice, but Chuck soured on that after a while. He wanted more; he wanted them to choose to adore him. When they still did (not knowing any other option), Chuck got bold and created humanity with free will too, only their mission to adore him would be made more difficult by him inflicting suffering upon them. If humans still chose to worship him despite their miseries, that would prove their worthiness as good creations.
Then Lucifer ruined the experiment and became the first ever being to betray Chuck’s grand designs. And by doing so, he instilled a millennium’s-long fear in his father of being challenged/proven irrelevant/replaced by one of his own creations. So Lucifer was exiled and scapegoated for humanity’s “sins” (i.e., not worshipping blindly enough), and Chuck exiled himself because he was that shaken up by the possibility of beings he created having the desire to defy him. His whole “great plan” for the apocalypse was created, in a sense, to rewrite Lucifer’s betrayal by introducing a new character, Michael/Dean, who chooses to stand up for Chuck of his own free will and smite the defiant son once and for all.
Like… it’s so interesting that Chuck has no desire to insert himself in his own story of the end times, but it makes sense when you realize that it was never about him reasserting his own authority—it was about him being chosen. The plan goes wrong of course, but not because Michael/Dean betrayed him. It was just Lucifer/Sam again (p.s., I love him seeing the boys as one in the same, just a personal hc). It’s a setback but not a worldview-shattering disaster.
So, Chuck lets the world keep spinning in its own chaos just to see if it will find its way back to the perfect ending, and it’s not until Jack that Chuck finally starts sweating for the first time since Lucifer’s fall. Because this is a child even more perfect than his last favorite—and it wasn’t even created by him. Jack is the child of the defiant son, a descendant of Chuck’s power but just removed enough to not owe his existence to him. This is a true threat. Chuck is scared shitless of this kid, the kind of scared that makes you not want to even acknowledge the scary thing until it’s necessary.
But still, Chuck won’t get his hands dirty. He wants his perfect model son (Dean/Michael) to choose to destroy the threat on his behalf. And when the perfect son doesn’t comply? That’s when he really loses it. Loses the desire to be chosen that’s guided his every action since the beginning, and now all that motives him is petty retaliation. Chuck will not go gentle into that good night—he will rage and destroy his every creation for no other goal besides destruction. At least when it’s just him and his defiant sons left, he will remain the most important thing in the world to them.
Needless to say, being left to age in a small, irrelevant human body is a fate worse than death for him. Being replaced by Jack, even more so. It’s his worst fear realized by his own actions to avoid it.
