If anything I feel like most don't get why it's really as bad as it is. It's not that it opted for a Tale of Genji style ending that bothers me, that in of itself isn't the problem as far as I'm concerned- it's that it's poorly executed and entirely unconvincing; there should be some critical impact that is flatout missing from the reveal. What bothers me most about it is it simply dropped a huge bomb that Rin sees Daikichi as a total stranger, and has done so for the entire 10 years they've been together, with no fallout at all regarding it. Daikichi himself, who only viewed himself as her father and spent the entire first five volumes growing into that role, should be devastated to suddenly find out their familial arrangement was entirely one-sided. I don't think comes entirely out of left field: she spends the entire series calling him by his name rather than some familial title and from it's pretty obvious a few volumes in after the timeskip considering her musing about her future all revolved around her being content to forego a family of her own and just taking care of Daikichi as he gets older. But those alone aren't really enough sell me that she never attached herself to him in a familial way when otherwise they only ever acted as such- he was her "father" and she just grew up into being a daddy's girl. So instead of what I feel should have been a huge blow to the guy, instead there's just muted and weak reaction, followed by him simply not resisting and letting her do as she wanted and everyone else isn't even phased. But honestly, the entire back half of the series was overwhelmingly weaker than the first 5 volumes- it's not just this fizzle of an ending that was the problem, nearly all of the story content was shifted to a much less compelling subject: Rin having all the same teenager problems we see in every other teen shoujo manga. Her going behind his back to track down her mother and meeting her was good, as was the one callback moment where she calls him out on him claiming she can't get too big for a hug from him (during a moment where she was particularly distressed) back when she was still 6; but that's about it.