Oh I believe Ant-Man 3 was the inflection point. DS2 and Thor 4 were divisive, but I think there was still enough of a “let’s see where this is going” among fans. Black Panther 2 was also well-received and performed well for a sequel to a movie that over performed expectations. Plus this was the big, epic debut of the new Thanos, so you have people invested enough to see him in action.
Then it came out and everybody realized, “okay Marvel’s bullshit now”. Majors’ domestic abuse allegations also hurt fan morale too imo. Like obviously Eternals and stuff ruined the excitement too, but Ant-Man 3 was the last straw. For Marvel, and the superhero genre as a whole.