I'm sorry, that's so hard, but glad you're feeling positive about bigger picture.
I've been mulling over your post, because I have been through serious illness and had that disappointment of people who just brush you off with 'oh, it'll be fine' or whatever. It's quite a lonely feeling realising the only person you can rely upon is yourself and does anybody even care what happens to you. That's what distressed me more than anything, that sense of aloneness in the world and vulnerability more than anything specific.
But at the same time, I've been through a really traumatic experience with a loved one who had cancer, and I don't cope very well when other people are going through it now - however, reflecting as I read this thread, I don't think I've done the 'oh, it'll be fine' to other people because I am very aware of how much that would hurt them. Even if I can't step up I am more considerate with my words than that.
I tend to try and be the best friend I can to them - rather than distancing or disengaging - and then crumble in private. I know that I am not able to support people as well as I wish I could though. Certain aspects of it probably are too triggering for me to be as supportive as I'd like, but I would hope I wouldn't end up being so casual as 'hope it went ok' several days later - because I do still care.
I can imagine though that other people who step back or get it wrong do so because of their own traumas rather than because they don't care.
It's hard. When someone disappoints you like that you don't always know if it's because they're an arse, they don't care, they don't have any comprehension /empathy of what you're going through, they do care but can't cope, they care but don't know how to support...
Somebody once said to me that some of the reactions you receive to serious illness are more about the other person trying to make it ok because they're so frightened and are trying to make it ok. They become over-casual about it and almost dismissive because they're trying to make it seem like a smaller and less scary thing. But that ends up hurting you.
I think it does change friendships, but it doesn't have to end them (unless they have been deliberately / outright callous like some examples here). Some friends are great for any crisis, some friends are great for particular crises, some friends are only great when things are great - so you just learn to match your connections to the situation, but yeh, some turn out not to really be friends sadly.