OK, final thoughts after rewatching the last ep a few times and some other eps as well.
First, why do so many people call AB world "purgatory"? Purgatory is a sort of hell-lite dreamt up by the church because they didn't really believe what the Bible said about sins having been truly forgiven. So, depending on how much sin you committed in your life, you'd have to spend a certain amount of time in purgatory to have your sins purged before you were good enough to go to heaven. AB world is nothing like that, it's a limbo (but not in the Roman Catholic sense) between one life and another. It's where you free yourself of your attachments to your former life, in this case your regrets, before moving on with the next one. It's far more Buddhist than anything Christian. (And yes, I have recanted my former heretical belief that AB world was a virtual reality for the living, but it is still evidently a place created by something like the Yggdrasil system of the Oh! My Goddess universe.)
The main thing though is that Otonashi and Kanade
shouldn't have been there. Otonashi died without regrets (although he couldn't remember that at first because his heart had retained his final memories), and
Kanade wasn't even dead. Let's look at Kanade first:
- We already knew that she behaved so differently from both the "standard issue dead human" and the NPCs that the SSS had assigned her to a third category
- If she had simply died after a sickly childhood with an unfulfilled desire to thank the person whose heart she had received then she would have arrived in AB world quite naturally, but instead she says that she "got lost and wandered" there because of the sheer force of her desire
- She thanks Otonashi for his gift of life, which would be a somewhat meaningless thing to do if she were dead
As for Otonashi, he eventually got dragged there by Kanade's desire (confirming that the timelines of the real world and AB world are completely independent and cannot be compared).
So, back in episode 7, Otonashi says that he can
keep on living if somebody thanks him, and now, in the final episode Kanade unknowingly fulfills this prefiguring by thanking him. She disappears and resumes her life (no time actually having passed in the real world while she was "AWOL"); Otonashi, having now no reason to stay in AB world, disappears shortly afterwards. And now the miracle occurs (this is Key, right?). Otonashi resumes his life too - he doesn't die in the train wreck, but to allow Kanade to survive, her heart condition is cured. Because neither of them were supposed to be in AB world in the first place, they retain some vestige of their memory of it - Kanade sings Iwasawa's song, and Otanshi recognizes it. Good end. And it really can be a good end, because now their relationship can grow naturally rather than being skewed by Kanade's debt obligation to Otonashi (not to mention that it also avoids the otherwise insoluble problem of "2 people 1 heart").
One last thing - I realise now that Kanade is so small because of her heart problem. It's a common side effect of some childhood cardiac conditions.