Of course not, but is staying the same ideal? The story continues after what, in any other story, would have been the final act. Life goes on after the credits roll, and we get to see it. I like that.
Well, she was the main character. How can you have a story about a serial killer without the serial killer actually be in it? And that's her in the cover of the mange.
But she wasn't the main character? The story has been told through Kurosu's perspective since chapter 1. Being on the cover doesn't mean they're the protagonist.
She's certainly an important character, but the story isn't from her perspective. Kurosu, the main character, has been deeply affected by her, and we are seeing those effects in the continuation of the story.
This story won't be the same without her