> That in turn means that the existing feed specifications serve reasonably well to publish new items but very poorly to notify subscribers about changes to existing items.

That's a jaw-dropping misunderstanding. RSS items are not the content; they are notifications for something else. The content is elsewhere. When you have some content-object that changes, those changes appear as a train of RSS items. The RSS item is not the thing that it's about.

Your expectations and mental model are wrong, not the feed semantics.