JUST A LITTLE BIT OF NON UNDERSTANDING

Varekk said:
Shouko doesn’t die, she goes back to sleep.

After Sakuta said goodbye to Shouko near the intersection, she disappeared, passing all of her memories on to her younger self.
After Sakuta decides to choose Mai’s happiness over Shouko, He and Mai decide that they are both willing to start over from scratch if it gives Shouko a chance to live, but they don’t know how to actually do that.
Shouko then, having just woken up with all the memories of all of her selves, realizes that Sakuta’s life would have less pain if he had never met her, and so she goes back to sleep, projecting herself back to elementary school, and lives from there without indecision, finishing her “future dreams” assignment happily. From there she takes Hayate straight home from the park instead of waiting around and meeting Sakuta, therefore removing herself and her problems from his life, but also never jumping back from high school and meeting him on the beach.
That was important for him to come to terms with Kaede’s situation so it would be bad if it never happened, but luckily the nature of this form of time travel sends out fragments of the future being changed, to those affected, in the form of dreams.
Sakuta meets high school Shouko on the beach in his dreams and learns the same lessons from her, and Mai’s vague recollection of the previous timeline manifests in a smaller way, urging her to play the main role in a movie about heart disease, and since *THE Sakurajima Mai* played the main role, lots of people went to see it, and were persuaded to pick up organ donor cards.
More people with organ donor cards meant more chances for Shouko to get a transplant, which she does, which leads us to the final scene.

TL;DR Sakuta and Mai don’t go back to fix stuff, Shouko does.