Just finished Air and normally I'd go out of my way to make a review but since I'm a lot more interested in its ending which requires a lot of spoiler-ish talks so this would call for analysis instead.
To cut down to the chase, from how I see it, the Summer segment set up the foundation for how the world of AIR actually operates. There's no doubt that supernatural and fantasy are on the menu here, so stuff like reincarnation, or after-life, etc. is to be expected.
The most important factor to take into consideration is the fact that Misuzu, the central point of the story, is not quite a "separate" entity, but merely belongs to a greater being, namely, the Kannabi no Mikoto, or Kanna. I mean how else can you explain the "my other self is in the sky" stuff that she always mumbled, right?
In a sense, we can treat Misuzu as part of Kanna's subconsciousness. Kanna is locked up in the sky, forced to witness memories and dreams of mankind for a lengthy period of time. She cannot move, but she can however channel her will (or subconsciousness whatever) into a (randomly picked?) proxy underneath the sky throughout various generations.
Something I only discovered after watching the anime adaption is that the "memories" passing down between generations thingy WORK BOTH WAYS.
Yes, it means that they don't simply flow in a one-direction manner but the descendants in later generations can also share what they see in their life with the ancestors (this was briefly hinted by Kanna when she was at the village with Ryuuta and Uraha).
To be more specific, Kanna talked about how she saw herself in a "weird" outfit, surrounded by friends, her mother, bla bla, which is easily what happened in the main timeline of the story like you know, all that slice-of-life romcom high school stuff.
On top of that, winged beings are not of the same origin as humans whatsoever. Consequently, Misuzu does not belong to the realm of earthlings, it was inevitable that she had to part way with the people she had known for a long time and return to where she originated.
Ultimately, the girl and the boy at the end of the story inherited the memories (or will) of the winged being and Ryuuta + Uraha (remember Uraha has that bootleg Jedi power crap lol) so in a sense, they kinda "foresee" what was going to happen to their "predecessors", hence the seemingly confusing warning coming out of the boy's mouth.
With the curse being lifted, it's safe to say this duo would be the last iteration of the winged-being and the guardian, and perhaps they could proceed their life as they please without being affected by all that memories overflowing or worse, the corruption of their mind and body.
All said and done, though they never quite provided an elaborate explanation on the mechanism of the winged beings, I think this is more or less a plot device to help achieve a kind of "happy" afterlife ending.
I mean, don't overthink, it is Key we're talking about. They might have the resolve to kill off their characters but there's always a trick, a gimmick behind that to act as consolation for the audience or some sort.
Usually, I'd find this being a lame and ineffective approach but just this one time though, the build-up justified the ending. The whole Summer segment was dedicated to laying the groundwork needed for that bittersweet ending.
And I think it worked out well not just from a tear-jerking perspective, but also from a thematic standpoint where regrets are resolved.
Kanna never got to achieve happiness with her mother in the feudal age, later got her wishes fulfilled via Misuzu-Haruko circumstance. And goddammit, the final shots featuring Haruko and Misuzu are legit one of the purest things I've seen in a while, not to mention that banger of an ED BOI.
Additionally, the relationships between the "pair" of each route were also mended after the hardship they had gone through, which is a nice touch-up for the whole visual novel.
If anything, I'm glad for once it's not the power of love and friendship, and not even romance has its place here LMAO.