I considered many possible endings for this work, but the Schrödinger's Shizuko ending was definitely not one of them. Congratulations to the author for surprising me.
I could even accept this ending, which doesn't give us explanations and only shows that the protagonist overcame everything, if this were a manga that focused on the protagonist's personal development (like Hoshino me wo Tsubatte, which many people hated the ending of, but I liked), which it clearly isn't. The protagonist barely has any development; his conflict in the story is basically a "I won't date, I will date, I won't date, I will date, I won't date" (quoting the Brazilian philosopher Seu Jorge). Even the development he could have had by discovering everything that happened (Takuto being in love with him and having a fake relationship with Shizuko, and Shizuko being sick and possibly dying) doesn't happen because he doesn't discover any of that. He gets over the fact that his crush and his best friend disappeared, but he doesn't even know why. There was no development whatsoever. The same goes for Shizuko, who is the female protagonist and the great mystery of the work; we never see her point of view. Everything we discover about her is at the end from Takuto's point of view, but we never actually see her perspective on everything that happened, what she thought when making the decisions she made. The big revelation that she is ill is made in a dialogue between her mother and Takuto, without her present. Surprisingly, the only characters with a cohesive arc in this work are Umimi and Takuto. Not necessarily good arcs, but they exist. We actually see Umimi's point of view and understand what led her to act as she did, and we see her accepting her feelings for the protagonist and how that relates to all the craziness with her mother and stepfather (I found her stepfather's arc awful, but that's beside the point). Her arc has a beginning, middle, and end. As for Takuto, we see what motivated his decisions, his regret, him confronting his father and seeking redemption; he has a complete development arc. Unlike Riku, who knows nothing of what happened and we only know he overcame everything because there's a two-year time jump stating that he overcame everything, and in Shizuko's case it's even worse, we have no conclusion regarding her, we don't know her thoughts about everything that happened, and we don't even know if she's alive or dead. The story simply ended abruptly without developing its two protagonists. This manga could easily have 4 or 5 volumes and it would have been much better, because there's a lot of filler in the middle. Yako, despite being charismatic, is a completely unnecessary character. The whole story about Umimi's stepfather could have been cut (I would have appreciated not having to see that scene of her masturbating with the protagonist's hand), and they could have developed the ending and the main trio more, given a conclusion and development to Riku and Shizuko, and given more importance to Takuto throughout the story instead of sidelining him the whole time only to have him appear at the end and reveal all the mysteries of the manga before going on a journey of self-discovery. Besides the fact that the author pretended that many things didn't exist, like the hospital card that Shizuko's mother lost and Riku found, the mark on Shizuko's neck, and that countdown to graduation that at a certain point the author simply pretended never happened. By the end, it's clear that the author got fed up with the work and gave up.
Having said all that, I found this manga very fun to follow. I was eager for each chapter, I read comments about the work from everywhere possible, I saw the version in various languages—Spanish, English, Portuguese, and even Japanese—this manga drove me crazy. Is it a good manga? Definitely not, but art is about feeling, and this manga definitely made me feel things.