I genuinely loved this manga, but this bonus chapter is, uh.... not really that great. It makes things uncomfortable to me. Like, a child brain is a child brain that functions like a child brain no matter whose memories you stuff in there. So this sort of upends the previously cultivated relationships in the main story, and if it doesn't, then that's just morally reprehensible. And I don't find this scenario satisfying. If we had the knowledge that Sato-kun was living comfortably and happily elsewhere without getting involved directly with the main characters, that MIGHT be better, but I don't generally enjoy dropping in reincarnation as a fall-back in stories when it hasn't been introduced early on.
This is just a 'what if' scenario, though, so I don't think it's supposed to be taken so seriously (clearly that doesn't stop me from talking about it, though lmao). I'm just disappointed, 'cause there were plenty other satisfying 'what if's that could have been written and drawn up that doesn't screw with the story itself and its emotional beats.
Good thing this is just a bonus, though, 'cause I can just pretend it doesn't exist.
Thanks for reading my rant, whoever you are.