But although the post has generated a great many thoughtful remarks from readers on both sides, it took a quasi-anonymous comment from someone called "Nj_Librarian" today to bring out a point I've never seen made before:
When the Friends of Narnia were speaking of Susan and why she was not there, they didn't yet realize that they and the Pevensie parents had all actually been killed in the real world.
That fact wasn't revealed by Aslan until the next-to-last page of the book. Before that, the Friends thought that they had just been magically transported to Narnia again, like all the previous times, and that they would be sent back at the end of the adventure.
So they don't know that Susan has been permanently bereaved and left alone. Had they known this, their discussion of her might have taken on a different tone.
And s/he is absolutely right. The Friends of Narnia's startled reaction on the second-last page when Aslan tells them that they are all dead makes plain that they simply hadn't been thinking of their situation in that light. As far as they were concerned, Susan had simply missed out on another trip to Narnia, which was a shame and all, but perhaps there'd still be a chance to persuade her before the next time. They had no clue that there wouldn't be a next time, and that they'd already had all the chances to plead with her that they would ever get.
I can't believe this hadn't occurred to me (or, apparently, anyone else on the thread), but I'm very glad it's been pointed out now. Thank you, Nj_Librarian, whoever you are.