Are you really a "fan" if you don't want the back stories that form half of the narrative? It takes away from the experience if done poorly but the movie did them amazingly, integrating them together with the characters' perspective and thoughts. How many of the "fans" actually complained about the flashbacks? What percentage of the comments on the movie was about the flashbacks?
AI art or animation isn't going away no matter how much people hate it. As much as it dilutes the artform, it makes it just that much easier for any random person to get inspired and create their own animation. What happens when AI reach the point where you cannot distinguish between Hollywood movies and AI content? When you can create your own Avatar movie? When you can save your favourite character from their most undeserved death? Does fanfiction animation need to be hated too?