This is a question best saved for topic about Key in general, not just Kanon, so it wouldn’t necessarily fit the bill of a Key Point. I’ve chosen to discuss it here in the hopes of keeping things related to the topic at hand.
Still, I think the amount of magic realism used in Key’s endings simply stems from the author’s choices on how to end them. Maeda has gone on record to claim that his writing style is inspired by works with a good deal of surrealism and magic realism, such as Hard-Boiled Wonderland and the End of the World. Since he was the head scenario writer for Key from AIR all the way up to Little Busters, it’s only natural that his tendencies to use magic realism in his stories would seep into those works. Rewrite didn’t have as much Key Magic in it because Romeo Tanaka is known more for complex philosophical messages that don’t make much sense to the reader than magic realism. Not that it stops Yuuto Tonokawa from following in Maeda’s footsteps in this regard.
I only speak as an author-in-training here, but I’m also of the belief that very few, if any, writers actually like killing off characters they’ve grown to love. Key Magic is just a simple means of emotional relief for both author and reader, although most of the time it’s executed pretty poorly. In my opinion the best Key stories are the ones where there is real loss, perhaps not of a significant character but something very important to them. I won’t name those routes here, but I feel like Key could step up their game with their future works. Perhaps Harmonia, 2nd Beat and whatever new game Key’s coming up with next will strive to change things for the better.