I think you didn't get it. The last part of the final chapter and especially the "I'm not Seno", gives closure to everything. Even his best friend can tell he moved on.
Oh I totally get it, it was more than obvious, but I don't really mean that, oh and apart from that, the fact that Hakushima thought about it, does not mean that Seno did. Certainly there is closure, my opinion is, that it was a plain and dull one, in comparison to everything Seno went through.
And not necessarily a bad ending either! The author works with what they have and the fact they were able to make 10 caps is a lot.
When the main guy (I forgot his name) tells Seno all of his plans for the future, that's when Seno gets he's loved.
In the extra chapter Seno is shown as more honest with his bf, without fearing being dumped, that shows that their relationship progressed. Either way, since Seno didn't grow up knowing what love is, maybe he will be insecure for his whole life even if is showered with love.
Yeah, it was actually a beautiful manga, despite how it started.
While it was a happy ending and Seno found his happiness like most comments here say, I feel like that extra needed another extra, in which they do show us Natsuka showering Seno with love (and not just showing it with a sex scene). Like Seno actually confirming that Natsuka does love him now, not only acknowledge him now, but like him being in love for real, and that is not Hakushima anymore. This finale only insinuates it, but after everything Seno went through, I think he deserves better than that xD Showing them living together and Natsuka putting Seno's feelings at ease would have been a better ending. That's just my opinion tho