That’s not exactly the point though, and this links very well with your mention of Afterglow, which indeed invokes an image of hope, but also highlights a darker truth.
That section of the route isn’t about Mai and Yuuichi together. In some ways it isn’t even about Mai. It’s about Yuuichi. Throughout the route Yuuichi is stubborn and incapable of understanding Mai, but he lives as if he understands everything. This causes pretty much every dispute in the route.
So when we get the “Yuuichi figured out everything that was happening” section of the VN, where Yuuichi yet again believes he understands Mai’s situation, we’re once again shown that Yuuichi is in fact missing crucial information.
It’s not the ending of the VN, despite what Yuuichi initially frames it as, because Mai hasn’t even begun to come to terms with herself.
And the Afterglow is an interesting signal of this, because Afterglow is always the signal of Kanon’s what-if scenario. It brings about a dream-like conclusion that isn’t realistic but that Yuuichi chooses to believe in. It can be seen as a way of running from reality, and in every instance it feels like an extension of an already foregone conclusion.
To Yuuichi, and to the reader by extension, this is where the story should end… But to everyone else in the world, ending it there would be selfish. Yuuichi is the only one to have gained, and he didn’t even have to respect other people to do so. None of the details about Mai’s history matter to Yuuichi or to the reader, but it matters immensely to Mai. The entire struggle of her life was born from those events, and she in such a deep refusal of her past and her self that she’s perfectly willing to knowingly kill herself.
Yuuichi’s just running from anything serious, as he always has. Even at the end, when he imagines a future he could’ve spent with Mai, he is truly lacking in understanding of other people, and if it wasn’t for a wish bringing the world into Afterglow, Yuuichi would’ve failed to do anything.