After a while Superheroes tend to reach a status quo where it becomes almost impossible to implant any meaningful change into them. It becomes the job of the supporting cast to add new ideas into the franchise. If a franchise is about the hero exclusively it tends to become stale over time.
Examples.
I feel Batman has supplanted Superman as DC’s main hero mostly because their was definite push on the creators part to flesh out Gotham and everyone that lives there. Though they have been around almost as long as each other, I’ve always felt that Gotham was a more concrete location then Metropolis.